Get a friction drive kit from ThatsDax. Be aware that many have tried pocket bike engines. Problems with broken clutch springs is an issue. You don't want to be stuck miles from home with a broken clutch spring.
An option is to buy a 37cc Huasheng engine or a Japaenese 2 stroke and buy a ThatsDax kit. You can buy a Huasheng from Live Fast Motors. DON'T BUY THEIR FRICTION KIT - IT IS CRAP! Don't be tempted by the cheaper prices on their kits, you'll only get what you pay for - headaches!
Be aware that person size has a big impact on speed, as well as roller spindle diameter.