Yard sales and second hand type stores are a good place even the city dumps for the older style of frames but a welder will make the hard ones fit. One of the guys even fitted a full suspension bike with an engine its very cool more work than I'm willing to put out for a bike. check out the bikes here and you will get ideas to help you get going.
Welcome to the dark side of motorized bicycling.
One important thing brakes are very important the side pull type seem to work the best as they apply more power and have the biggest pads I don't have a disc braked bike but I'd bet they are very good. Your levers need to be mated to the type of brakes you are going to use if you build your own the old style pinch levers will not work very well on the newer side pull levers they have a diffenent geometry built onto them and will make it a booger if you get the wrong levers on the wrong style brakes.