there are many answers to this question, the short answer being what everyone else has already said: unless you know what you're doing and have at least 1k to put into the motor, prop it and leave it alone, that being said built and modded 951's can run a very very long time. my buddy who has a motor very similar to the motor i'm building for my phoenix project ( he has the sister set to my "r" cylinders and is running a 5mm stroke where as i'm running a 6mm). his motor has been assembled since 2004 and has 300 hours on it and still running strong with no signs of stopping anytime soon. and he's not gentle on that ski either, its in a 99 xpl and runs 71 mph.
when it comes to the 951 no single component can be replaced and huge gains be acquired, and often another change (or sometimes several) is needed make the engine stable again. An example being: say you found a pretty new pipe you wanna put on your engine a coffman or a rossier. now you can't just bolt that bad boy up, go the lake that weekend and hammer down on the throttle, if the engine will even run you'll have it close to blowing if not blow within 10 mins. you have to make carb adjustments, and in the case of stock 951 carbs, no matter how big a jet you put in the main it will not reliably flow more than a 1.0 and nothing you can do will change that. so unless you like blowing pistons if you do a high performance pipe you have to find an aftermarket set of carbs. get the idea?
now what can you do?
as others have said a solas concord 15/20 and new wear ring ~ 200 bucks if you get the impeller used.
new set of reeds(stock or carbon tec) ~ 100
richen the carbs up ( those things are already running ridiculously lean from the factory to meet emissions, not really gain any performance but will gain reliability)
mild porting and polishing, lower case and mono block. you can go more aggressive but only if you rebuild the whole motor will require carb tuning . ~400
reed spacers ~75-100
"white" 951 pipe and manifold. will require carb tuning and actually finding a real one, not a painted knock off, not worth it imo as more performance can be gained else where for the same money. but still an option ~300
if you rebuild the whole motor you can get a stroke put on the crank but now your talking major money. ~1000
also if you rebuild the top-end and go more than 1mm over stock you need to have the domes milled to match the new piston size, other wise you're just promoting more detonation and 951 are already very prone to that. that's why now days when you seen a backyard warrior trying to build a 1050 ( stock 951 stroke with ~92mm pistons) they barely last 10 hours, you have to match dome size to piston size otherwise your not taking full advantage of the bigger pistons and promoting detention.
imo the way to build a performance 951 that will last is to stay as close to stock bore as possible, stroke the crank, port and clearance the cases, port the mono block, aftermarket carbs, stock pipe, stock head, increase compression to ~135 psi and use oem seadoo parts as much as possible .
