Can't rebuild yours? I would think if you were going to go to diff block, a bore job or careful measure of bore and new piston, rings, and rod would be good idea. OR, can't just bore yours if bad bore and do same to it? Cubs have diff pan and don't bolt down on flat pan like many, that is one of biggest issues with an engine change. A valve job then of course and all new gaskets and then rebuild the carb while it is all down also.
One of the head bolts broke and I really messed it up trying to get it out. The block on eBay is from a cub just a 125 not a 122 like I have but it was the closet age wise I found to the engine I have. I'm going to check in the parts manual for spec numbers. The seller says it will work but I didn't know how to find out for myself if it will. I do have a service manual for my JD112 which has the same engine, that should help me with getting it all together. I am going to ask for a picture of the bottom of the block.