If you are getting a spark outside the engine, you are almost certainly getting one inside as well. There are all sorts of reasons why this won't ignite starting fluid, one being no compression, so check it first. If the carb is already providing too much fuel, adding extra with starting fluid isn't going to help, and if the carb is providing very little or no fuel, there may not be enough fluid to get it to fire. More esoteric problems include slipped valve timing or slipped ignition timing, but check compression and clean the carb out before thinking about those.

 

 

Compression should be over 90 psi. As I wrote, don't think about slipped timing etc, until you have good compression and a clean carb.

 

 

First thing with low compression is check the valve clearances. If there is no clearance on one or both valves, that is going to drop compression. If they are OK remove head and cylinder. Do not buy any new parts until you know what is wrong. If there is no major scoring to piston and cylinder, either check the bore diameter yourself or get someone else to do it. Check piston diameter as well. If both are OK check the gap between the ends of the top ring when placed in the cylinder. If this is over 0.020" all you need is new rings. Check if the bore is standard so you get the right rings. All acceptable size limits are listed in the workshop manuals, it is time to get one.

 

If you can, have someone do a leakdown test on the motor. Basically you fill the top end with about 100psi at Top dead center compression (TDCC) and then measure how much air is leaking out. If you truly have 50psi compression (you did have throttle open right?) then you will have a lot of air leaking somewhere.
At this point listen at the exhaust
Listen at the carburetor or airbox
listen at the oil fill hole.

This will give you a good idea of where your problem is without tearing anything apart.

If it is intake or exhaust then you may have lack of clearance on the valves and just need to adjust them. Or bent valves or burnt valves which will require a tear down to fix.
If it is good out the exhaust and intake but it is the cylinder (oil fill) then you will be diving into the piston and cylinder.
DO NOT ORDER ANY PARTS UNTIL YOU KNOW WHATS WRONG.
Sorry to yell but everyone loves to buy first and figure things out later and that often just leads to you wasting money.

History on the motor?
Has it ever run for you?
did this problem just start?
has the motor been apart before?
Did you submerge the motor? Water cannot be compressed so bad things can happen if you sucked it in.
Have the valves ever been adjusted?

 

Can't see the point of a leakdown test. If the problem isn't valve clearances, which is the first thing to check, the head and cylinder are going to have to come off anyway, and you will find what is wrong then.

 

Leakdown test will tell you if the valves seal even if you have clearance. So say you check the valve clearances and have some but you still have air blowing out one valve, this lets you know you have a bad valve or more likely debris between the valve and seat which will at some point burn a valve.
If you do the leak down test with the cam chain off you can check the piston at most any point of the bore that will tell you there is a piston seal issue.

Yes in either case to fix it things do need to come apart, the reason for the test is so you do not have to tear things down and replace parts for no reason. The head gaskets on these bikes are cheap but that is often not the case so I am used to trying to not spend customer $$$.