All posted before were good suggestions, especially the Harley motor. I guess if you already haven't done it the first thing I would check would be the carb, I know you said you had adjusted it but it might be internal. Also it would be a lot cheaper to do a tune up on the carb then an engine swap. Seems to be doing the same thing my buddies Sears was doing whenever it was under a load. It would idel and run fine but once it was under any sort of load it would bog down and die or act like it was starving for fuel. Tore the carb down and did a tune up on it and it runs like brand new and has no issues while under load now.