5-7HEAVEN: for the cheapest price, (if you're wanting actual Schwinn) is to look for Schwinn Stingray Frames from the LATE 70's to early 80's, Those Schwinn frame for 20 inch came out in 1963 and the early year frames are pretty Salty (especially Krate series)
or an even cheaper route, look on ebay or heck even walmart for those purple Brat's bikes, there's a guy here on the forums that has put an engine on one,
side-note don't even try to put an engine on a schwinn fast-back frame, wasted about 2 weeks on one when I tried it, but that's how I like it though, live and learn and have dirty hands in the process....
good luck