So many whiners! How sad.

Anyways:
First you need to clarify if this thing will be legal to use wherever you are. You don't have your actual location in your profile, so I couldn't help any with that.

You need to go to your local car-registering-license place and ask them "if I put a motor on a bicycle, can I still use it on public roads?". Don't ask about a 'moped' because [a bicycle with an engine you put on it] may not automatically legally be considered the same as [a moped] (that was manufactured with an engine on it).

A lot of whatever follows hinges on if this would be legal or at least practical to try to use or not....
Even if it's not legal you can still do it anyway. There's lots of horrible things in the world today, but constructing illegal motorized bicycles isn't one of them.

This forum has a motorized bikes sub-forum, but it isn't much good for gas engines.
Motorbicycling.com and motoredbikes.com would have more info.