I dont mean any offence to you on this, but your not only talking about two completely different sleds but a completely different era of snowmobiling.
To answer you question, no it cant just swap in, and no its not worth it. There is much better solutions to your delema. A newer used sled is only going to cost a grand or at most 2 grand more than the motor you want to put in that sled. Put it this way, I bought a 1995 polaris xcr 600 with 2000 miles in 1998 for $2000. I got a pretty good deal but really, in the spring time these deals are all over the place.
I just want you to answer the question of why would you spend the money and time to have a snowmobile that isnt really much of a snowmobile as far as the 2000's are concerned. Buy a late 90's sled and be extremely happy with spanking your buddy and having a load more suspesion capabilities and much better handling. But keep your Tundra for another 20 years and sell it as a classic sled to someone who will pay a pretty good dollar for it.