Sounds like it could have seized up. With the carb swap you went through tuning it to get the proper air and gas mix for the bike? A lean bike feels like a beas up until it does not have enough lubrication and heats up the cyl until the piston gets swelled and sticks to the cyl walls. Usually it is the rings that bloat into a port. Remove the head and see what the walls of the cyl looks like. If you "soft seized" the piston should still move. If the cylinder is still smooth then you could still run it. If it feels gouged then you will need to hone the cy and get a new piston. Or a new whole top end. If it is hard seized you will need to replace the whole top end as all the pieces are probably not salvageable.