it's up to you and your budget. A 2 stroke costs $200ish for the kit delivered and a decent 4 stroke kit is $370ish delivered. The 2 stroke might have some issues w/ tire clearance using the kit motor mounts on the chain if the tires are 2.35 or wider. the 4 stroke has some more adjustment for chain clearance. the 4 stroke kit will require using the wider pedal kit to clear the motor. You might want to share what mechanical / welding fabrication skills you have. I like the gasbike.net kits myself. Im sure thre are several other sites to get good deals on motors. The eBay kits are hit and miss. The hardware on the rag joints are suspect on all kits. You might also want to share your budget to get some feedback ALSO what look are you going for ? vintage ? boardtrack ? gonzo ? rat ?
A 2 stroke motor has a clutch lever and you pedal the bike then release the clutch to start it. A 2 stroke bikes require you to mix oil with the gas. The 2 stroke bike is more affordable than the 4 stroke. You must have some clutch skills to operate and run the 2 stroke.
The 4 stroke kit costs more and features a centrifugal clutch. There is no clutch lever and you start the motor using a pull rope. The 4 stroke motor runs on straight gas. You might find the 4 stroke motors a little more reliable however they do cost more to repair in some instances.
best thing I can say is lurk, ask questios and check out the builds you see on the site. Ask questions from the builders most will tell you everything they know in order to help you out. I have built 4 bikes myself and love it. I hope to build 500 more at least. If I can help you out, give me a shout....... Good Luck !