As long as it's not made in China......oh waiT
Unless you're buying billet motors ( fuq that), price is insane for just one motor for pleasure riding, racing is something different. Everything else is chinese. Get the right factory and company, you get a quality engine. Not that things don't happen. Uf you ar eblowing motors every 7 months, you are A) building race engines, and 7 months on a race engine is insanely awesome, or B) buying the cheapest left over reject crap they have out there for sale or C) have no clue what you are doing with these bikes, or D) learning and experimenting the very expensive way. My buddy the preacher, his 12 year old daughter maintains a flying horse 66cc, has since she was 11. Was her 11th birthday present. I taught him, he taught her, she keeps it going. Even wrench her dad's bike, which I built identical to her bike, except I gave him a smaller sprocket. His does 40, her's does 30. Her's is quicker to full speed, his is a little slower to wind up. both flying horse single piece kits. He bought them, idk where, but I guess he knew the guy selling them. Both bikes are out an about a lot. Even come over here to go to the race track parking lot where we race around for hours. Guy's, if an 11 year old girl can change a base gasket, check a plug, knock a chain, clean an lube, etc, etc, with no previous experience, whats up? Please read some of the post here about maint, and performance. tons of good stuff. If you do know what you are doing, and don't be shy if ya don't, read, buy a better quality engine. we all had to learn sometime. Read up like you are starving for info, let it saturate the brain fat. The brain fat loves to be used, more ya use, more you gain.
edit addition: I just wanted to add, this little girl shows no interest in making some performance engine bike. She doesn't wanna learn porting, port cleaning or nothing, I think thats why her dad went with the flying horse. No need to port when you change a jug, which she can do all on her own, from start to finish. Bearing, pin, rings and all. Single piece jugs are great for people like her. She's just extremely proud to have and maintain a bike where all her friends have only a pedal bike. Give's her bragging rights at that age. lol. She does however have her dad text me a photo of the jugs when she is doing maint on the bikes. See if they need replaced. If they do, they order them up and slap them on. Makes me weep a few tears of joy ( which my beard proudly hides, to see a little lady wrench on things. lol.
Try a flying horse engine. They aren't the most powerful, But they are good enough and last a long time.