The .91 fs engine will be an outstanding choice for this plane, you will love it!!!
I had an this combo in mine and it was awsome, no need for extra buildup on the firewall, just put it in.
You will have to cut the cowl for the engine, but it looks so cool with the cylinder head hanging out the
side of the cowl like a big muscle!!

Watch the balance, and try to keep as much weight aft as you can to offset the engine weight.