now that you've got the top end off, check the rod bearings. Kinda difficult without tearing down the whole motor, but a couple quick checks should tell you whether you should continue before you buy a new piston and bore that cylinder.
Inspect the top (small end) of the rod, needle bearing and wrist pin. Shouldn't be any scoring, esp in the rod bushing. A little discoloration of the wrist pin is normal, but if it's blued from over heating, you'll need to worry about the rod too.
Push the crank to the bottom of it's stroke. Grab the rod and hold it upright.(
You should not feel and up and down movement, and no side to side. A little wobble is normal.
Repeat at 90 degrees all around to check further. If there's any play though, at the bottom of the stroke is were it's usually at it's worst.
I'd grab the flywheel and check for play in the mains there, no wobble and no in 'n' out.
If you really want to go all out, pull the clutch cover and check that side too.