Abstract: A personal watercraft is disclosed as including an engine and a forced air induction system including an aftermarket supercharger. The aftermarket supercharger replaces the personal watercraft's original-equipment supercharger and increases the charge of compressed air provided to the engine. The preferred supercharger incorporates a spur gear that includes a smaller pitch diameter than the original-equipment supercharger. The smaller spur gear and engine flywheel thereby provide an increased gear ratio and permit greater supercharger rotational speeds when compared with the original supercharged engine. In order to permit intermeshing engagement with the engine flywheel, the spur gear is axially offset from the original spur gear axis. However, the supercharger case maintains the same engine-mating surface as the original-equipment supercharger so that the case may be received within a complemental socket of the engine housing.