If you wish to obey Pennsylvania law concerning motorized bikes, you will have to keep your engine under 50cc. Your first decision will be to determine if you want to drive the front wheel, or one of the rear wheels. Friction drive is the simplest, cheapest drive system, but other systems may be better and more elegant from an engineering standpoint. With friction drive the gear reduction is accomplished through a small engine driven roller pressing into a driven tire, thus eliminating a gear box or some sort of small sprocket driving a large sprocket via a chain or belt. The roller will wear the driven tire a bit faster and works poorly when wet. As long as the roller is exactly perpendicular to the tire, wear should not be extreme. I suggest you read the sections covering friction drive, frame mounted systems, and rack mounted drives, as well as the section covering MB laws in Pennsylvania. Look at the photos in the gallery as well, there are some very nice bikes and trikes in there. Once you've decided how you want to go, then call a few vendors to see what they recommend and to get an idea on prices.
