The honda 330 piston is 76mm. That with the 86 mm stroke of the 500 is around 390cc.
Not sure how you would sleeve it. I bet a company like LA sleeve could help.
Probably end up being cheaper to run a 250r motor with a 370 kit.
Could do this, not sure if the wrist pins are the same, but a custom rod is nothing.
Just would need to check to make sure the rod doesn't hit the piston skirts and it could work.