Those are really heavy all steel made sleds.My neighbor has one that had engine problems.He put a 340 Arctic Cat engine in it.We tore the original engine down and found that it had melted down pretty bad.It would of needed a complete overhaul and possibly a new crank.
340 cat engine will bolt rite up but the wiring is slightly different and the clutch will need to be swapped over from the original engine.
If it's been sitting for a while Don't try to run her without completely rebuilding the carb,replace the condenser and clean or replace the points.It doesn't take but a couple minutes of lean running to either burn a hole in a piston or cook the rings.If it gets hot enough to turn the header red then it's hot enough to take the temper out of the piston rings.The rings go flat and you gould up and seize.Any 2 stroke will do this but snowmobile engines are more susceptible to this because they sit in storage over 1/2 the year and the carbs tend to gum up.
They are nice old sleds but they are really really heavy.Try not to get it stuck to many times lol.