My two cents:
In this month's Sport Aviation on page 75 there is a new completion of a CriCri which has empty weight of 194 lbs and uses 22 hp. Since its design intent is aerobatics at fairly high speeds, it doesn't meet part 103 regs, but it does show what can be accomplished with an aircraft in that size range. Here is a video:
The part 103 rules make an aerobat very difficult, not because of weight, per se, but because of top speed and stall speed. On the other hand, how about a design of a tiny motor glider? Many things are possible if the MTOW is low enough, even on meager horsepower. I don't think it can be done with a 254 lb craft carrying a 250 lb pilot, though.
"Big whorls have little whorls, which feed on their velocity;
little whorls have lessor whorls and so on to viscosity."