Hi Michael, First, don't let the naysayers get to you. Time was, ultralights were quite dangerous. Nowadays, they're somewhat safer than riding a motorcycle on public roads, though still more dangerous than driving a car. As flydude said, a lawnmower engine probably won't cut it. The typical ultralight engine is 25-55HP, and is most often a 2-stroke as few 4-stroke engines are light enough. Rotax engines are the most common; while they originated in that company's snowmobile engines they've diverged and now the aircraft versions are purpose built. You probably won't need the TIG. Aluminum aircraft structures are rarely welded; the welding destroys the heat treat and few amateur builders have the facilities to properly reheat treat an assembled structure. Steel tube you can weld; aluminum you rivet. There are few all aluminum ultralights. One is the Hummel Ultracruiser. The fact that you are asking such basic questions in a forum like this leads me to believe you aren't very familiar with ultralights. Rather than jumping into a new design, you'd be better advised to learn to fly an ultralight first (the fact that no pilot certificate is required does NOT mean no training is required!). Flying around for awhile in a used ultralight (which will almost certainly cost far less than a new original design) will put you in a much better position to start working on a new design. An excellent online resource for homebuilt aircraft is HomebuiltAirplanes.com.
