i really want to motorize my bicycle plzz help?

[graig]

i want to put a mount above my rear tire and run the chain downwards from the engine to the sprocket which is the simple explanation. im having trouble with the type of engine i should use because i am doing it to a 20" bmx bike. i want to gear it moderately so that i can make it up hills alright but also go at a reasonable speed. what kind of engine would be good? i was thinking a pocket bike engine thats 47cc but idk if that would give me enough power. the bike doesn't way too much mayb 50 lbs. i was also kinda thinkin about a horizontal lwan mower engine. can anyone give me some ideas of engines that i could use that arent too big or expensive but powerful? thanks