A '93 RS250 seat units will fit, with a bit of cutting and few ali brackets. I've fitted 93 fairing and seat to an old MC18.......can't for the life of me find a picture of it......but it does work.

I've fitted 2001 stuff to an MC28 as well, Fontyyy had it installed on his bike to, but both of us used a Tyga rear subframe (I think he had my one) as the '01 seat unit is a little narrow to fit on the std Honda sub frame.

You have to remember that the '93-2004 seat unit is quite long on account of the RS250 frame being about an inch shorter that an NSR one. You'll therefore need to cut about an inch allround out of the front of the unit to make it sit right on an NSR. If not, the unit hangs too far out over the rear wheel and looks pony.

Good luck if you do decide to go the RS250 route.