Long ago I learnt the hard way that beefing up a kit to make it stronger, does not make a stronger plane. I built a David Boddington designed Ultimate and could not believe the small sizes of wing spars, 3/16" X 1/8" and the lack of ply doublers etc. Finished weight with MDS 148 was 9 lbs for a large Biplane. But it survived many cartwheels and withstood the most violent airobatics. A super light plane has little inertia and can fly slowly.
I agree that a 91 should fly your model with a fine pitch large prop.