I grew up with carts. As I remember, you can just buy a good B&S or Tecumseh in the right configuration cheaper than the lawnmowers they usually go on. I also wracked my young brain trying to figure out how to use the chain and derailer from a 10 speed to create a "transmission" allowing at least one "overdrive" gear. It only had "friction" brakes that were like flaps that slapped down against the tires, but you could still lock them up in a panic. My set up (I was like 7 years old) would do 40 and was awesome fun. It wasn't a racing cart though, those are insane crazy fun. I ended up finally selling it when I was like 17. Kinda wish I still had it, really.