It's not the motor so much as the rest of the ski that will make it go faster. The key to these machines is moving the water through the pump. I would start with a variable pitch impeller 14/17. A rideplate will get you to a plane quicker. A scoop grate will ensure that there is always water in the pump. These are the cheapest mods that will give you an increase without breaking the bank. After that it is carb, exhaust, and ignition, all of which are fairly pricey.