Co Yam has a very valid point about the MM motor vs the Viper motor. The MM motor will give you thousands of hassle free miles, they may no produce the same hp as a Viper motor but this is a back up/spare sled you're talking about.
The Viper motors are subject to melting down in the mid range or just tooling down a trail at a constant speed or throttle position which causes a lean condition. This can be overcome with spot on jetting and also running a set of EGT's to kep an eye on the burn.
Chassis wise they're both running the same tunnel, approach angle, rear skid and bulkhead. The only real differance is the Front Shocks, Trailing Arms, Upper Radius Rods, Plastic (hood, Belly Pan) and Seat. All of which can be interchanged if so desired
I have a 01 MM (w/144 Camo) and a 03 Mtn Viper (w/156 Camo), they're both tunneled w/ Drop n Rolls with very similar chassis set up. The MM motor is untouched except for a Bender Can, and the Viper has a Bender 835 thats been ported and running Bender Triples. Both are extremly fun to ride in just about every snow condition but I favor the Viper when the POW is deep because of the xtra HP power and track.