As a sanity check on that drag calculation, a stock "2hp" moped does about 30mph. I've calculated about 1.5hp (at the ground) for going 42mph - not totally out of the realm of possibility, considering that 2hp moped might do closer to 35mph with a gearing change, and I assume 2hp means at the crank. If anything, my estimate is a bit conservative, meaning my power estimate may be sandbagging a bit.
In any case, this methodology should be good enough to compare better than the butt dyno. (edited)