im going with you have low compression do to running lean and also high possibility air leak, you mentioned someone not too bright rebuilt it, we see funky hose rigging, your running no exhaust coupler so your back pressure is all outta wack..... everything seams to be there tho, just need minor touching up and replace a few things and you will be in pretty good shape.
do you know if it still uses the oil mix tank or was that deleted and your premixing your gas? iv known ppl to run straight gas in a premix vehicle and after 3 days fry the top end with the same results your having... bought it running, as you keep poring in straight gas the engine is slowly dying till poof.