The 50cc Echo you can get a short block as the SB1080 and they are around $190. This is the biggest of the plastic frame Echo chainsaws.

The 60.3cc, 68cc, and bigger chainsaws are integral crankcase/frame engines. You cannot get a short block. You have to buy the crankcase/frame, the crank/rod assembly, and then the top end. Then you have to cut the saw frame from the crankcase.