KTM parts interchangeability... I should have written my master thesis on that shit instead of renewable energy like I'm doing right now!!!
- Most KTM offroad(ish) bikes use 190mm width triples, this goes from 125-990 Adv. BUT it also includes 450-560 SMR bikes with radial caliper mount.
- KTMs street-ish bikes like LC4 Supermotos, both the old 640 version with the axial gold 4 piston Brembo, and the newer 690 SMC/Duke/SM, use 210mm width clamps. This also includes 950/990 Super Dukes and Supermoto.
- Next you have to be careful with offsets, because generally all offroad-ish bikes have leading axle forks and therefore less offset in the triples (generally 18-24mm, only SMR 14-16mm) but more offset in total, while the street-ish bikes have more higher offset triples (around 35mm) but less in total due to the in-line axle. The exception is the 640 LC4 SM, it has wide triples with 18mm offset and a leading axle.
I have no idea what offset would work well for you Pete, but if turning radius is a concern with a 48mm WP conversion on the Ninja frame, then it might actually not be a bad idea to put on a 690 SMC triple clamp and radial fork combo. This gives you the wider spacing and the most triple offset but less total (35mm IIRC). The disc gets spaced out with the factory 8mm spacer, so it should be possible to get enough clearance to the spokes of a 21" wheel with a radial caliper and a 320mm disc (just not the Brembo I think).
Edit: you can still swap around wheels with the wider triples, they all use the same hub width. On the right the extra space gets taken up by a wider thick part on the axle, and on the left you simply change the left spacer.
Hope that was clear as mud...