Ooh! Them heads are beat up!
I noticed in the 4th picture, the head on the right,by the exhaust valve on the bottom next to the seat looks like a vertical crack... Is it?
the cylinder looks good from what I can tell, except a few score lines, if possible take it to a machine shop and have them mic both, and hone them,
shouldn't cost more than $50., And measure the crank journals, although they look excellent, it's cheap insurance from all your work blowing up.
Lucas oil assembly lube, good stuff, coat all the cam and crank the,crankpin and valve springs.
look forward to seeing your work!!
The Yardsport YS400 is a mix of parts from the Yamaha parts bin. The manufacture is HiSUN, who also happens to manufacture parts for Yamaha and more OEM's. The bottom end is basicaly the same as the Yamaha 350 Grizzly/Bruin. The top end is more of less off of a Yamaha Kodiak 400, early 2002 style. I would start by using OEM Yamaha parts for the clutch. 2007-2011 Yamaha Grizzly 350 wet clutch. the parts are of higher quality than the Chinese counterparts. This might be part of your problem. They bolt right on and will work and last much better.
Update: it came off fine, just a few taps with the rubber mallet. Once that was out of my way, I was able to assess the carnage, pretty bad, was not a rod issue as described after all, looks like the screws that hold the air filter housing on rattled into the engine and banged up both chambers, and pistons. Both intake valves had closed on 2 of the screws, other chunks of piston. Not all bad as the case is fine, save for some light scoring that should clean up in one cylinder. The crank, and both rods seem fine, I need to check the intake valves again, both seem like they may be usable, not sure yet. The heads took the brunt of the damage and will cost the most. Pricey parts. Will see if I can get them tig welded and repair the chamber surface as they are pretty bad.
Some more pics