Ok I have a 2004 400ex it has the 440 big bore kit wisco pistons web cams. Well about a year ago started leakin lil oil around the head so had some backyard guy work on it. He said he fixed it took it out soon as I got it back. started leakin a lot of oil so took it back to him he said the head was cracked but it's hard to believe the head was cracked because it ran really good

 

What work did he do? You engine needs compression to run well, and the crack and leak oil, but if not a crack of part of the compression chamber would still run the same. Get some good engine cleaner and really clean up the engine, figure out where it is leaking from and post a picture. That would help a lot to determine likely cause. Could be as simple as a head gasket, which once again can leak fluids but not compression.

 

Being air cooled the ex's dont handle a 440 kit well without cooling mods. Its common for them to pull head studs. HD heads studs are available.
You might get lucky and find out whoever did the 440 kit used chitty gaskets or did not torque properly. Clean it up real good and give it a inspection, pics help.
BTW, was the new piston stock compression 9:1?

 

Thanks I'll take a look at it this weekend and also think it's the valves to because when I let off on the throttle it blows little white smoke