The main 2 types of engines are chain drive (usually the cheap china HT kits for around $200) and the friction drive which has a 78mm clutch where the engine mounts into the side of the drive. There are also kits like the EZ q-matic and the grubee 4-G. Here is a vendor that offers all 3 : http://www.piratecycles1.com/skyhawk...gine-kits.html
Hope this clears things up there are a lot of variables some guys get real creative...really comes down to how much you want to spend and what type of bike you are going to build on. Most are built on beach cruiser type steel framed bikes.