You can't do much of anything to the pump. You either run the stock pump or you change the hull to accept another pump. Some people have modified the hull to accept a 750 or 800 pump, but they are all 140mm so the only benefit is being able to use SXR props, that is a pretty involved job.... You can modify the hull to accept a superjet pump but that is a LOT of work and not worth doing for most people. The only things most people do involves selecting the proper prop for their motor and maybe changing the tail cone or the diameter of the nozzle.
The pump should match the motor, so WFO is right, motor first. You can not get a lot of power out of the 650 motor, but being a person with a mod ski, I'd vote leaving it stock unless you have a LOT of time AND money to mess with it, and don't plan on riding as much as you work with it. Very few combinations fit in the hull well and any motor other than stock is going to be a lot of work to obtain results.
However, if you want to do that work, you have several options with the motor. You can bolt in directly a 750 or 800 Kawi motor. This is very easy to do, however the exhaust pipes for these larger motors are much bigger than the stock 650 and you start creating a lot of problems very quickly trying to get it all to fit in the hull. You may also fit in a Yamaha 2-cylinder motor which will require 300-500 in parts (not including the motor) to be able to use the yamaha motor with the stock motor mounts. Some have put larger 3-cylinder motors in X2's but it's a major job and that is left to but a select few.
My opinion is that for most people who just want to enjoy the X2 the best option is to leave it stock or to modify the 650 motor. The next option is using a larger Kawi motor but it's a hassle. The most power is going to be obtained from a yamaha motor but that is probably also going to be the most expensive.
Go to the X2 forum (the forum you are in to read this post) and click on the sticky post at the very top that says "X2 mods".
For you, considering you mentioned a minimal budget, assuming you have a running 650 X2, I'd say you should buy parts to modify the stock motor. Start with a pipe, then maybe second a carb, then after that the power per dollar decreases quickly. Do nothing to the pump but buy an aftermarket prop to match your motor and enjoy the thing. Pipe $200, carb $100, prop $80, $380 total and that's quick simple easy upgrades. Anything more is going to cost a fair amount of money and be a lot of work. If you have extra money, spend it on upgrading the stock steering, maybe an intake grate and ride plate.
I own a very modified 750 X2, and if I could go back in time, I'd gladly trade it for a stock 650 X2 with a steering upgrade. It's been a long time getting my ski to where it is and almost no time riding it, which completely defeats the purpose of owning it.
