I have an S90, not a G90. They are a little different due to some different internal parts (crankshaft and piston for sure possibly the rod.) and I think the S90 is timed a little bit faster. I haven't ran it much because my little SI3 with the Jett .35 has been my goto for a run at the park. I suspect to see 50-55mph on a good day from the big SI3 maybe more. I'm using a tuned muffler not a tuned pipe. I don't think the .90's really benefit much from a tuned pipe because they're not timed real high. You will get a boost in power but not as much as say a S61 ABC. They can't breathe well enough due to having a .90 piston/liner in a .60 case. The gas passages are much smaller in the .90's.
Liteply over foam isn't likely to be strong enough for the .90 especially on a pipe IMO. If you incorporate some aluminum for strength and fiberglass the tub then maybe. I wouldn't be running anything bigger than a .60 on a wood over foam build personally. I build with balsa frames, and various thickness birch plywoods. I can stand on top of one of my riggers crap sponsons with one foot and it won't crush, crack, or break. My little SI3 .20 series with the Jett has run 49.8 with a 3" rudder and wrong CG, and that boat is 5lbs RTR. I expect to see 60+mph this year. I think 4.5-5lbs is unrealistic for a .90 sized build since the engine weighs about a pound with muffler or a little less as it is. So if the engine is a pound, that gives you 3.5-4lbs for the hull, control rigging, electronics, paint/finish, etc. I wanted my SI3 .20 to come in at 3.5-4lbs and I'm glad it's heavier. It'd be airborne by 40 otherwise.
This is all my opinion, so don't be offended that I'm not totally on-par with your ideas.. I love the idea and can't wait to see what you build, but I have to look at it from a realistic point of view. Weight can be of a benefit to a point on fast boats.