Bearings, ball joints, springs and seals. That's all parts that require replacement with age, but usually get neglected.
Rebuilding clutches. A lot of performance is lost here, but people don't realize it unltil the sled gets un-rideable.
Change the pull rope. It's gonna break someday.
Look over the wiring harness. Anywhere it rubs, after thousands of miles can be worn through.
Grease everything, then grease everything again, and grease it after that. Skids in particular have lots of movable parts that get exposed to water. Failure to grease them will result in seizing, loss of comfort and performance during a ride, and eventually egging out of holes requiring repair or replacement.
Idler wheels. They have sealed bearings and you can't grease them, but they do wear out. They need to be checked a couple of times a season. Failure to do so, will cause drag on the track wasting horsepower, and eventually loss of the wheel which is more expensive to replace than a bearing alone.
Yes, you guessed it, maintenance is the most important place to spend money on an old sled.
After all the maintenance is done, then think about performance. [thumb]