Having recently purchased an O.S. .91 FS, I can recommend it. Easy break-in and lots of power. The papers say use a 12 X 10 for break-in, So I ran that the first 2 tanks. I've since been using a Bolly 13.5 X 6, and the engine isn't loaded down at all. I've put just a bit over 2 gallons of fuel through it, and haven't had any problems.

I've got the one without a pump. Probably the pumped one would be better if you are putting the fuel tank a long way from the engine. My tank is directly behind the firewall.