Gas Powered Micro Monkey Bike?

I have a Micro Monkey Bike and I want to put some kind of small gas engine on it, maybe a GoPed type engine or something, but I really have no clue about putting an engine on anything or engines period, all I know is it will make my monkey bike go. I was just curious if anybody had any suggestions on how I should go about doing that. And I'm definitely looking for the simplest, most cost effective way to do that. Thanks!