Hey all
Not up to speed on NSU stuff. This is a 1 rotor right? I think they used a 6mm seal just shorter than 12A?
The best option, if considering ceramic seals, is to find other owners of like engines to keep the cost down. If you are using cermet housings you will want ceramics.......cermet loves ceramics

Even if you are not ceramics will be kinder to the housings.
Cermet housings with 80,000 miles on NRS 3mm 2pc: