it can't....
Well.... maybe it can.
The only way to safely reduce it's speed is to limit how far the throttle will open. So... if you made a ring that would go over the grip, and only allow the throttle lever to open so far... that would be the only way I would know to do it.
You don't want to try to build a throttle stop at the carbs because you will wind up breaking the throttle cable... and there isn't a way to do it electronically on a 2001 ski. (a 2004 could have a learning key built that will limit the speed to 25 mph)