You would need the 20 HP.
My choice would be a twin as the load is distributed between 2 cylinders as opposed to 1, sounds obvious but a single cylinder engine of this age of design would be under too much stress in my opinion. I don't know what revs the higher power single Listeroids are turning at but it must be much higher than 650 RPM so the benefits of a slower running engine are lost. The twin however can turn slower for the same power so less stress , noise, lower fuel consumption, lower wear etc.