If you can afford it an EZ Q-Matic is the way to go. I've never owned one but the reports coming in from users are all good. They do run $600. If you do the math tho even spending $600 on a kit pays off pretty quick fuel savings rack up quickly at 100+ MPG. It really depends on several factors what you choose to buy. I have ran several of the cheapest China kits and am very happy with their performance. These things do take some mechanical aptitude if you aren't a fairly decent mechanic I'd go with a thatsdax.com friction drive with the supertitan 50cc engine gonna run ya around $400. Hope this helps. Dax has an add here in the left column. If you go with the cheaper china kit here's my vendor of choice http://www.piratecycles1.com/66bienkits.html
He may not be the cheapest but shipped to my door $200 is a fair price and I know this dude stands behind his products.
You can expect to spend 8+ hours installing the cheap kit less than an hour on the friction drive which is way more reliable and IDK how long it takes to put the EZ Q-matic on but it has a transmission that extends your power and speed range. Good luck! Have fun!