They sell cylinders for clamshell motors alright. You can still have a good rod, crank, bearings and a base cover to transfer over to a new cylinder. It's a good bit more work to swap a top end on clamshell designs too. But I would not open a clamshell motor if I had the option to just transfer the whole thing into a chassis.
Tell me about it!! I pulled the 390 apart when the decomp valve fell apart and jambed the piston to the cylinder. That was a pain to pull it down. That was also my first major saw dissasembly!
