For 90 bucks for a replacement 66cc engine (including shipping), BGF has the best price. The closes thing I've found is livefast's which I have on my bike right now (I have a bgf and livefast that I swap if something breaks on one engine while I fix it).
The livefast is much better quality, but the BGF is about 40 bucks cheaper I believe. I recommend you buy a box of 10 replacement 6202 bearings and replace everything from bgf on arrival bearing wise. I've had all four bearings go bad in the first 800 miles on my bgf engine.
Here's a link to the bearings.