I'm leaning more towards a trailer wheel or similar. The hardest part is making sure the wheel fits a common highway tire size. I've found cheap go kart brake calipers, which will be good enough for now. I can machine my own spindles better than what I've seen on go kart parts websites, and I can make brackets for a disc brake to mount up easily enough, and probably the disc too. I'll have plenty of time to do that this winter.
Oh, on a positive note, someone identified my engine as a 16HP briggs flathead. That explains the large torque at low speeds. That's plenty enough horsepower to get me to highway speeds without any trouble. I also have a close enough model number to get replacement parts if I need them.