When you set up an unknown boat, it's always best to err on the side of caution regarding steering. If you set the rudderr up with about 30 deg each way, it may be too much so you may well find that you won't be pushing the stick so far over to turn. This can then be modified by reducing the throw based on how much the boat turned relative to stick movement. If, on the other hand, you set up too little throw, at best it will take huge circles and at worst will be up the bank :-
The 2227 prop you have (as it's ally) may well be a submerged drive prop! The 22 refers to the diameter which is 2.2" and the 27 is the pitch. If there is a thread in the boss (usually 5mm), it is for a solid propshaft and will not be suitable for surface drive which normally have a 5/16" straight hole with two notches at the front which engage into a 'dog drive'. Another way to tell is, if the back of the blades are straight, it is surface piercing, if they are convex, it is submerged. I suspect that it's a "Propshop" or "Mocom" which has just been "chucked in" with the sale.
To confuse you even further :-\ Octura props are available in plastic, beryllium copper or stainless steel. The diameter is the SECOND pair of numbers and the pitch the first pair ie. 1447 where the 47 is the diam in millimetres and the 14 is the ratio of the diam to the pitch ??? ???
Prather don't make plastic (i think) so they are only BeCu or Stainless. The sizing code is beyond me even though I actually use them :-[ They are numbered 220, 225, 230, 235, 240 etc.
To cut to the chase so to speak, your SC91 is a 'sports' engine which will probably not be tuned?? If so I would suggest one of the following, Prather 260 (2.52" x 3.9" pich) Octura X465 (2.56" x 3.58" pitch) or better still start with a couple of Octura plastic (GRP but not the hard GRP) which you can experiment with (cut down) until you get somewhere near.
As a point of interest, your tuned pipe is an early PMC pipe, with the water cooling jacket covering the front expansion cone. It looks, from the pic, about a 40 size. If you can let me know the diameter at the widest part of the cone, I'll tell you what it's for. Irrespective, if it will go over your header (manifold) you could still use it although it may be too small. Using a smaller than optimum pipe will still give some improvement to RPM as long as it is set up to the right length. Looking at the long pipe at the front, I suspect that this is how it's been used (and with the cost of a new pipe nearly £100, who can blame them). Your most difficult task, I think, will be to get the manifold from the side of the engine, round to behind it, then through the tunnel at the rear. You may end up having to make your own - but it's not that difficult, if you can silver solder.
Hope that helps and sorry for being so long winded
Danny