Spotted this cylinder & piston on Ebay because I'm in need of these pieces.
Seller says it's 68mm bore. Isn't the 300cc cylinder 72mm? Would this be for a
Isetta 250 or 600 even? I'm not sure, does anybody know?

Trevor:

The flat finned side of the cylinder indicates it is for a fan-cooled BMW
engine, and the 68mm would be for the 250cc. Probably it came from a 250cc BMW
Isetta or maybe even a 250cc BMW industrial engine if they made one.

There are probably several reasons you won't be able to make this cylinder work
on the 300. Try to find a used cylinder less than 73.00mm in diameter, and then
have it bored to the next suitable piston size. If that goes without a hitch,
order the piston. I once had a cylinder bored to the next size, but a hollow
still remained in the cylinder wall and I chose not to use that cylinder.

If all goes well with your engine rebuild, it will hopefully be the only Isetta
engine you'll have to rebuild.

tinker 295