A 13-13 would, of course, be far too much load for a fan .90. Somewhere between 8 and 10 inch pitch should do, at 20,000 RPM or so in the air, and 10 to 11 inch diameter, depending on how slippery the model is. If a Nelson .40 can pull a pylon model at close to 175 mph, a fan .90 should pull a model of the same drag at well over 200. I used a flat-topped fuselage, with a fiberglass or molded thin plywood half-circular section cover to enclose the pipe and clean things up, and let most of the engine cooling air flow over the pipe and header, then discharge near the rear.