Yes, it will fit. I have made hot rod "stroker" Echos using a 25cc crank in a 23.6cc engine. The 21.2cc and 23.6cc engines are identical except for bore size, while the 25cc engine has a longer stroke and a physically bigger cylinder. It's been a while since I did it; I think you need a couple of spacer washers on the crank to center it in the crankcase and take up some excessive end play when swapping the cranks. You also have to make a cylinder spacer, but you size it to get the deck height exactly how you want it.

AV8TOR