If you want to do a custom project at some point in the future, you may also consider a larger displacement 4 stroke, such as the 79cc Predator available at home depot. The >100cc 4 strokes are mostly bolt-on applications, and all the parts you need are readily available, such as a motor mount and a drivetrain, referred to as a jackshaft, that will provide gear reduction. There are also other products available at www.affordablegokarts.com
. It's still not cheap, but nothing custom ever really is. But like I said, these engines are mostly bolt-on.