To install a starter, you need several things..
the starter itself, and mounts on the engine block for it
a clutch with the ring gear for the starter to turn
a solenoid to activate the starter,
an ignition switch to activate the solenoid
a battery to power the whole system, in a cradle capable of holding it
a rectifier to charge the battery
wiring to tie it all together
a stator with enough excess capacity to keep the battery charged
If the 93 XLT ever came with a starter, then it should be doable. But as you can see from the list, it's not a 20 minute install.
Grab the Polaris 96-98 service manual from this thread.. http://www.snowmobilefanatics.com/forum/topic/102341/display.aspx
Look at the schematic in the back for the 96 XLT (the wiring diagram is the same) and you'll be able to find out if your motor and witing harness have the necessary stuff already in place.
In my opinion, unless you're really committed, it would be more work than it is worth. YMMV.