Hi Jeremy
The prospect of fitting out a Huntsman with a petrol engine is an exciting one because they are great hulls and in my opinion ideally suited to IC power plants. In respect of using an air cooled engine that has not being designed for boat or marine use, I think you could be taking a risk of it overheating. If you could easily adapt it to include water cooling I would as suggested already. If not you could take the risk and initially run it for a short period looking out for any signs of overheating when bringing it in. Leaving the window unglazed will give a good flow of air especially when the boat is at speed.
In terms of the prop shaft I agree that a 5mm diameter shaft should be fine. If you fit a hardened spring steel 5mm shaft it should definitely be bullet proof. Alternatively for my smaller engine nitro boats I run with a non hardened 6mm shaft and they have proved fine. Oh, when you mentioned sub surface drive, I am assuming you simply mean submerged drive....? Assuming so, the three key things I would recommend are that a good quality prop tube incorporating a ball race at the end inside the hull is used. Secondly a good quality tough coupling and thirdly take care to ensure accurate alignment of the prop shaft to the engine.
In terms of Huntsman hull size, as mentioned whilst that size Huntsman hull should nicely handle your engine, do check that it will fit inside. The 48 inch Huntsman that I have, the old wooden kit version, has less room at the front due to the slope and curves of the deep V bow section than one might expect. The fibreglass version has more flexibility because you have more scope for fitting any reinforcement bulkheads around the engine as opposed to having to work around a frame and all the internal plywood panels of the wooden kit that I built. Air cooled engines not designed specifically for model boat applications can be rather bulky in size and also the output (crank) shaft will need to be sufficiently low down to enable you keep the prop shaft angle reasonably low.
Out of interest my Huntsman has an old Webra 40 nitro in it so it easily fitted inside.
In terms of prop size, I think you will only get a proper pointer if any members have used a similar engine and can share their experience. The prop that I use on my FSRV multi boat is 58mm in diameter running a tuned 26cc Zenoah. However you might find that your engine may be lower revving with more bottom end torque and needy of a bigger prop. I really don't know on this one. But you really do need an answer on this point as the last thing you want is to install the prop tube with say 30mm gap between its end and the bottom of the hull then find out you can't spin a bigger prop if needed! Conversely, you need to keep that shaft angle as low as possible for best handling of the boat and maximum speed and stability.
Hopefully the above gives some guidance. Do keep us updated on your progress.
Cheers
Craig