MC28 forks will also slot in, but you will need MC28 (or similar later threaded) calipers.
NC30 forks are the same diameter, allow you to use an NSR 3" or VFR 3.5" front wheel, and the bigger 296mm discs.
As Si says, you can fit just about anything if you want to though, once the NSR stem is pressed (or sleeved and pressed) into the new yokes. My MC21 has HRC RS250 USD forks with a custom alloy stem and my MC16 has 1990 HRC RS250 RWU forks (the '90 forks have preload, rebound and compression damping, but are shorter than the NSR forks) with HRC lower alloy triple and custom alloy stem. The '18 had to "make do" with an F3 front end, which is for the sake of argument, just a re-worked NSR front-end really, although it does have an alloy bottom yoke!
NC30 front-end with big discs (stick with the 3" wheel in my opinion) or an MC21SE front-end is probably your best bet for a cheap but good setup.
_________________
Andy.
NSR-WORLD.COM
Please keep all responses to Forum posts on the Forum so that others may benefit.
Please DO NOT PM me for technical advice. My time is precious, and you will probably receive a faster response on the Forum anyway.
Check us out on Facebook! Please "like" our page!