If you are lucky crank seals will make it run like crap. Usually when they go, you are riding across a lake at the farthest point possible from the trailer or home, the seal starts leaking, You notice the sled is running better than it has ever run before! A lean mixture will run really good just before the piston melts down and takes the cylinder wall out too.
A sled that has low miles, or been sitting for a few years even in a climate controlled area is still tough on seals. The seals can be 15 years old with small cracks in them just from age, crank shafts could have the oil lubricating the shaft/seal area go dry from sitting, or from being flooded. First time it is run, the seal could wear down quickly until oil gets redeposited from the oil\fuel mix. It just plain is not worth taking the chance for the small investment of a few seals and a bit of work. Same with the carb boots. I have seen brand new boots that buckle and leak once they are tightened down.
Same with carb needle and seat on the fuel inlet. Most are rubber tipped or Viton tipped. Once they wear or leak, after you shut down the sled, fuel will siphon from the gas tank to the carb (which sits lower than the gas tank) and if the needle and seat or the anti-siphon valve in the fuel pump afe not doing their job, you could siphon most of the gas through the carb, into the bottom of the motor, and out the exhaust. This will dry out the seals (of course) but even more, once you start the motor, the fuel in the muffler comes shooting out onto the floor and once the air\fuel mixture hits the right point, could make for a loud BANG. Luckily it doesn't happen too often as the exhaust coming out of the motor usually doesn't have much oxygen in it so it won't allow the fuel vapors to burn.
Moral is; New crank seals, Mikuni needle and seats, and make sure the anti siphon valve is working in the fuel pump. (should be a taller Invader or Intruder style pump, not the short standard pump)