Hey,
I have seen this dude that has customly put a 2-stroke petrol motor (similar to a Weed Eater or Chainsaw engine) and has put it on his push-bike. I dont know how he has done all of that, but he certainly didnt buy it of a bicycle store...thats for sure. Its heaps awesome of how he has done it and I was love to have one. I wish he made them and sold them to people around his area. That would be cool and then I'd buy one off him.