Making a 70% scale P-51 and bringing it in under 254 pounds is technically possible, but only if you are a university or a large aerospace company. The financial and technical resources that are required to succeed at this will be huge. Paul MacCready could have done it. Lockheed and Boeing can do it. One of the Technical Universities in Europe, or a German Akaflieg could do it. Our own BobK could do it if Bill Gates and Jeff Bezos both bring their checkbooks. The vast majoity of private homebuilders are not in the position to do it successfully. Then, once it is done, and built, and weighs 254 pounds, it will fly like crap. Not like a Mustang, not like an ultralight. If you drop a pencil accidentally, the pencil will poke through the wing skins or fuselage.The engine that you developed (from a clean sheet) will produce about 40 horsepower for 20 minutes, until is siezes or breaks, because the engine had to weigh 12 pounds. The tires will be about 8 inches in diameter and look ridiculous, the same way as most ultralights look like they are on toy wheels. I'm sorry to be a naysayer and a party-pooper. My agenda here is not to spoil your dream, but to prevent you from spending many years and huge financial expenses pursuing something that is unrealistic.