I was was thinking about buying a 80s rutan quickie q-1, it has a 18 hp motor. I want to get a ultralight but I like this one because it is fast. Can I get a motor that would be within the requirements? I don't know how I would be able to make this an ultralight. Iwas wondering if a Rutan 277 engine would work well in it and if this would be in the requirements
I doubt the Quickie would fly well enough at very slow speeds to meet the rest of the Part 103 requirements -- specifically, the limit on maximum level flight speed. Limiting fuel capacity is easy, and the 18 HP original engine worked well enough on the Q1 -- but it was a lot faster than 55 kt (close to double, as I recall). From comments (thanks, Gerry) I understand the Q1 has a stall speed of 42 kt -- fairly reasonable for a light aircraft, but well above Part 103 maximum of 24 kt.
You might be able to slow down enough by installing a flatter-pitch propeller, but that would have other effects (overspeeding the engine, for instance) that would need to be addressed, and then you'd be trying to fly with almost no speed range -- 42 to 55 kts isn't, IMO, a safe performance envelope (and yes, the Q1 would still go faster than 55 kt in a dive, but the level flight and climb performance are where the safety issues lie in this case).
Speed is not just about the engine. The reason the Q1 is so fast on an 18HP engine was that it had high wing loading, to put it a simple way it has to fly faster for the wings to produce enough lift. Its stall speed is about 41 knots, which is a characteristic of the wing shape and area, if you put a smaller engine on it you won't change the characteristics of the wing.
If it had a lighter engine, the wing loading would be slightly smaller. If it could be rewinged (longer chord and more camber?), then it might work. You could get enough lift at a slower speed. A bigger wing would weigh more, but perhaps that would be offset by the lighter engine. Of course, the CG would change and the wings would need to account for the shift in cg.