Also, vent the fuel tank up high. For the diffs some guys use bellows instead of routing the vents up high. The bellows will crack after 5 years or so though. You'll need to seal the clutch cover and housing and slather all of the electrical connections in di electric grease. Brush up on your electrical system trouble shooting skills, because you're walking into some training on that. After a year or so the corrosion will start raising hell with you, but you'll get good at figuring out whats giving problems. You'll want to run a new crankcase breather hose, put a couple of loops in it higher than the air box, when you get too deep it'll help keep water from entering the crank case that way. Oh, also put a little synthetic motor oil on the gas cap seal once in a while, it'll help seal it. Move the radiator and fan up high if you can. Put on your party hat, there's no better way to have fun!
I never ran the exhaust stack while racing, but for general deep water farting around I did. It saves a lot of headache if you stall or something. If the snorkels start to go under and I shut it off, it was nice to not have to worry about water going in the exit.
There is just nothing more fun than playing in the mud and water, no matter how old you are!