Total restoration is almost never "worth it" in terms of selling the machine for a profit. It can be worth it in terms of having a machine that few other people have though, it all depends on what you're looking for.
Raising sleds from the dead in north central MA.
'70 Ski-Doo Olympique 12/3 - The Yellow Bullet
'71 Arctic Cat Panther 303 - Wankle Panther
'78 Arctic Cat Pantera 5000 - Back from the dead!
'79 Polaris Cobra 340 - Back from the dead!
'95 Ski-Doo Grand Touring SE 670