I don't like friction mounts because they chew through your rear tire. Mine is a 66cc Chinese motor, and it starts every time. I only use it to play around the neighborhood, but still, it is very dependable.
The reason you didn't get a million immediate responds To your question is that every week, 2 new guys join the forum, and their first post isn't an introduction (like it's supposed to be), they just jump on the general discussion forum and start asking the same exact question very single week.
What's the best engine?
What's better, 49cc or 66 cc?
What's the difference between 66cc and 80cc?
Where is the best place to buy?
And the list of same old questions go on. Then they demand we answer them promptly, "wow, five days, a million views and only one reply? Thanks guys!!!)
If you would do about 15 minutes worth of research, you will see that a million people already asked the same exact questions you have asked. And you can probably get the answers from their posts.
And you will get a million different responds, too. Everyone will tell you that that is better, or this is better. Buy this engine, don't buy this..
I think we can all agree, though, is don't buy from Gasbikes.net or their associates unless you like getting ripped off. The negative feedback to positive feedback ration is something like 15-4. So, buy at your own risk.
I have a 66cc mounted in my frame (traditionally) and I love it. It's fun, and I have no problems with it, other then vibration. It has gone about 30MPh, but vibrated my rectum so bad I had to stop and have a rest before I turned around to go back. My buddy has a side mounted engine that has vibration only during idle, but is smooth at higher rpm. Mine is the opposite.
Either way, the fun isn't in riding it alone. It's also in building it. I had so much fun building my bike, it makes it much better riding it, because of the pride I feel of knowing I personally built that from scratch. I would have paid double the cash, and had half the fun if I got that bike pre-made.
Anywho, best of luck.
P.s. I got my engine and everything else from pistonbiks.com. I liked the fact that i talked to a human being on the phone, and I got a tracking number for my purchases. I know I can call them immediately if there is any issues. I recommend you check out their website.