the gsi came with the reliable 717 which made 85hp. there are pipes available from a company called factory pipe that gives it about 20 more hp, but that puts more stress on the engine, requires a higher rev limiter, and reduces the overall reliability of the 717.
check your local laws about pulling a tube before you do, most states require you to have a 3 seat rated ski, mirrors and a spotter. you also cannot have more people on the ski + tube than the ski is rated for, so no pulling two people on the tube.