You can get yourself a early XLT and you'll love it! I had a '94 XLT Special for 4 years and never had a problem with it, and i sold it for $1700, so you can get one real cheap. There is all kinds of motor mods you can get for that triple also if thats what your into, good luck!!!
That chassis has been around for a long time. There should be no problem putting different motors in that chassis as long as you buy the full size chassis and not the smaller one used in the Sprint. Much of the newer suspensions can be fitted under there without too much trouble although you will find the suspension is not too bad that was used at the time. As far as clutch goes that same P-85 clutch is still used today so lots of parts to play with. Carbs are standard fare. New plastic skis are a good add on item.
I have owned Indy chassis from 1980 to current. Up until the advent of CRC steering the chassis were virtually identical. With minor alterations the suspension can be updated easily. Current models have cosmetic changes mostly (hood and belly pan). I have a 94 Storm with several 700 RMK parts on it including an Extra 10. Engine swaps sometimes are a little more challenging especially the new twins. I would caution you about modes to an XLT. In stock trim they will last forever but pipes dramatically shorten crankshaft life and parts are getting difficult to obtain. I really like the Polaris line due to the ability to swap things on it so easily.
i just got done putting a 2000 rmk 700 engine in a 1989 indy 500 chassis, the wiring harness pluged right in and only had to change two motor mounts, i also put a m-10 in also and changed the front to extra 12.
I have a '93 XLT with (it's a long list) Tripple pipes, SLP mass wheel, SLP airbox, balanced clutch with Heelclickers, balanced driven with SLP spring and helix, Arctic Cat Powder Special handle bars, Wildwood racing brake with vented disc., rolled chain case (1" down 3" back), Bentwrench Performance tunnel, '01 RMK XtraLite rear suspension, Polaris 144x2" paddle track, Ski-Doo Summit rear bumper, Dial-A-Jets, '98 RMK plastic skis and Fox shocks all the way around. I did all the work myself and spent about $3000 in the last two years on a $1700 sled. However, it runs with every 600 I've come across and better than most. It has no problems staying with my brother's RMK 700 with a 136 track.
I've had no problems finding parts for XLT's and parts from about 10 years worth of Polaris's will generally bolt right on. The only thing I have left to add is a Holz front end and I'm good to go for another 10 years. That and an 800 twin :-)