SuperTigre's "X" series of engines were rear-exhaust engines, intended to be used with a tuned pipe. Some were designed for racing, others were just high-powered rear-exhaust engines. The X-45 was a rear-exhaust pattern-type engine. Racing engines were limited to .40-size displacement in the official classes. The racing engine was the SuperTigre X-40 Pylon.

You'd want to prop your X-45 so that it's turning about 13,500-14,000 RPM when on the pipe, probably with a 10 x 7 or 8 prop. The engine was well out of production by 1984 or so.