This worked becausee our forks and the RXV forks are the same size and produced by the same company.
If you are looking to improve the 658 forks you can try racetech gold valve inserts or you can try to rework the front end to accept a different fork tube set up.
What outside diameter are your fork tubes at both of the tripple clamps? Once you know that measurement you can see if there are some older dirt bike forks that might work. If there are then you just find a full front end from that bike - forks, wheel, brakes and bake lever (or use the BMW lever). Hope that makes sense.
As far as 21/17 vs 21/18 the 18 is more common. If you are having a set made why not get what most other non-BMW bikes have.
Ride more, bark less