You would have to move the rear suspension mount brackets. Also, the track will hit on the snow flap mounting bracket and the flap may get pulled in above the track. The bracket can be bent outward and a retainer installed to secure the flap, or add an extension plate. Or you could switch tunnels - check to make sure the attachment is the same first, I think they are the same. Then you could also use the 91 seat (if you have it and it is in usable condition), the storage compartment is larger.
If the 91 is not damaged to prevent it, consider switching the 89 parts to it. Beside the longer track, the ski stance is wider and the ski suspension is better. The brake disc is a slightly larger diameter, although I think the self adjusting caliper will work on the 89 okay. The handlebars are wider on the 91 and will have warmers, but the electrical connectors are different. I do not know if the 89 wiring harness has the warmer wiring in it. There is a generating coil under the flywheel for the handwarmers, if I remember right, it will not fit on the 89.