I *think* theres a 116" rubber track you can use with the drivers you already have. I don't know how you could go to a 121" without big changes to the tunnel and all the suspension.
A rubber track swap was fairly common on those for lighter weight and increased speed.
If more traction on ice is what you're really after you can stud the cleated track, drill out rivets from the cleats and replace with studs.
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