First of, I just want to say thank you for the wealth of knowledge on this forum. I've been lurking for some time and have learned a lot and had a ton of help from reading past threads. Seems like every time I google search an issue, I wind up back here.
I bought a barn find 92' LT80 for my son about a year ago. Quad was not running when I bought it. I parked it in one of my storage sheds and am now working on getting it going because he is now big enough for it. When I tore into it, mice had done quite a number on it. I installed a ton of new parts, admittantly some of the cheaper eBay alternatives. It has a new eBay top end kit, UNI filter, boysen reeds, NGK plug, NGK plug end/boot, and eBay carb and intake elbow. The parts were cheap enough so I figured it was better to replace everything, and more cost effective, than rebuilding the carb or replacing the rings. This all got it running good, but it had no power. Upon further research I dove into the clutches and found the front one to have some issues. The key on the crank shaft was worn down and the clutch spinning on the shaft. Upon disassembly, I found the crank seal was out. Now it has new seal, new clutch rollers, clutch inner and outer face(around the rollers), rear dust cover, key, and cone washer. While doing all this I disconnected the auto oiler and capped it all off. I trust premix way more. I took the rear clutch apart and cleaned it all up. It moves freely and I can force it open and let it snap shut. Also installed new belt.
The quad starts right up on choke, runs when taken off choke, idles great. When I hit the throttle it barely wants to move on it's own power. I have the clutch cover off and can put the back end up on a stand, hit the throttle and monitor what all is going on. Front and rear clutches seem to be working as they should. Front closes, forcing belt out, back opens up, dropping belt down. It revs up when the back wheels are blocked off the ground. When placed on the ground, and reved up, I can hold it back from moving with little effort. Won't even attemp to move with anyone on it. Just seems to bog down and make noise, but won't open up/make any power.
i know I must be missing something and it's driving me nuts. Especially because my son wants to ride and can't. That's the biggest bummer in all this. I can get pictures of anything if needed. Any help is tremendously appreciated. Hope I can get this thing going.

I forgot to mention that i replaced the petcock with a standard Banshee one. Also has new fuel lines and inline filter. I checked fuel cap and it's venting good. I'm going to tear the new carb down tomorrow after work and see what I can find. Even though it's new, it is Chicom, so probably has crud in it.
I'm not sure about the exhaust pipe, it smokes good and blows hard when back end is lifted and revel up. If it's full of mice debris I'm not sure how to tell. I did remove the drain plug on the bottom of the pipe with it running but no exhaust came out. Is that normal? Can I run it momentarily with the pipe off to test it?

So I finally got some time today to work on this some more. First, took the carb all apart and cleaned it real good with carb cleaner and compressed air. Took extra special care and triple checked all jets. All clean. Tried moving the needle E-clip up or down a few notches and re trying. No improvement. Cleaned out the old carb the same way, reinstalled, moved needle as well. Still the same issue, no power under load.
Took the exhaust off and burned it out really good and blew it out with compressed air. Banged on it to knock off burnt crud inside, not much came out in the whole process. Compressed air blows through it with no restriction, what goes in comes out the other end. No change, still no power.
I borrowed a compression tester from a friend and tested it, dry, and with a squirt of oil in the cylinder. Only goes to 50psi. It starts and idles fine. Revs up with no load, bogs down when under load and won't move itself. Seems like compression is ok if it's doing all that, no? Again, this is with a new eBay top end kit. Tomorrow I'm going to pull apart the mag side and check the crank seal there. I did the pto side when I replaced the crankshaft key and rebuilt clutch.

Ring gap... is that where the split of the ring lines up so it compresses correctly?

Well, this is what I did today...
Pulled the mag side apart and checked seal. It doesn't look bad, I'm not seeing any oil build up from a leak or anything like that. Dry as a bone and looks ok. I did notice what looked like some wet spots under the head, possibly from the head gasket. Double checked the torque on the head nuts, was ok. Tested compression, same 50psi. I took head off and re installed original head gasket with some copper gasket spray. The original head gasket is thicker. Checked compression, same 50psi. For the hell of it, I removed new top end and piston and reinstalled original piston, rings, and top end. Base gasket was ok, I applied a little more copper spray, same with head gasket. After reassembly I checked compression again, still 50psi.
Could this loose compression from the reeds or reed cage? I did install new Boyesen reeds, might have goofed something up. I guess that's the next thing to take apart, but I needed a break from it. Maybe this week. I guess I can order a new reed cage and mag side seal as well as new top end gaskets.
The rings on the eBay top end kit are weird, they don't seem to have a top or bottom to them. No markings either. Any thoughts?

Yeah this whole thing is throwing me for a loop. I was under the assumption that it wouldn't run at all with 50psi compression. When I took the reed cage off originally, the previous owner had globed some silicon on it to seal it, so that may be an issue since I cleaned it up.
The other thing is the compression gauge kit I'm using could be off, it is a pretty old Snap-on set. May see if I can get a hold of another one to double check it.
As far as the rings are concerned, I fit them in correctly with the split lined up on the pin. It was beyond a pain in the ass to compress them and get the cylinder to slide down over them. The original Zuki piston, rings, and cylinder are a tight fit too, but not as tight as the new kit. Another weird thing is the compression reads the same with all the old components reinstalled. I know the original rings are correct on the piston because I never removed them from the piston. Leads me to think that the problem is elsewhere, hence why I may look further into the reed cage and might as well change the mag side seal while I'm at it.

I have seen a bunch of threads with similar symptoms, rebuild topend and have low compression, but no solutions from the OP's. Only one thread had a similar issue and the OP got his head milled down .030 and it seemed to fix the issue. I purchased the cylinder and piston kit on eBay, with no head. I'm reusing the old head. Could the old head be warped or that far off? I did see some oily gas leaking on the bottom side of the head where it meets the cylinder, but when I took it apart the head gasket looked fine with no blow by stains or marks.

Is it worth it to get the head to a local machine shop to be taken down?