Re: GT50r - do not do it
Not sure what was wrong with your engine?
I got 2 similar Cag engines off eBay from Zero Pulse USA (Stage 2 Pocket Bike Engines) & they both needed new longer headbolts w/red loctite, copper base head gaskets (replace the paper ones), new spark plug (NGK C7HSA) & wires (using an 7mm automobile plug wire).
Both are equipped with Pocket Bike expansion chamber pipes.
These engines are very powerful & push both my bikes beyond 50 mph. I had a 98cc Lifan 2.5 on one of the bikes prior & this little engine has more go.
Like the china girl engines, they do require attention & tweaking which to me is part of the fun of our hobby.
Sounds to me like you aren't geared correctly. The Pocket bike engine needs a gear reducer to run a 32 tooth. I geared mine w/ 1.5" V-Belt pulley on engine & 22.5" wheel pulley (rim mounted to rim). I tried a 1.75" engine pulley & it wouldn't allow the engine to get into its powerband...Huge difference .25" makes...