WP43mm forks from a KTM should "drop in" the stock triples, but there is no right side axle clamp (fork bottom) with caliper mount available for them. If you want dual discs you'd have to go with 950/990 forks with 48mm slider diameter, but those will require some custom triples. Doable, but not cheap.
If somebody would tell me the diameter and thread pitch of the 2013 BMW fork cap I can check whether the cap (this houses the rebound adjuster knob and on some models also the preload adjuster) from an adjustable KTM WP 43mm fork - which houses the rebound adjuster knob and on some models also the preload adjuster - would fit.
Then the next question is if the BMW feet can be bored out to accept the adjustable compression holder, but I don't think this will be a problem.
After that it's only a matter of modifying the cartridge for the right length and stroke and fitting the right spring.
Proud member of the HUSABERG ADVENTURE TEAM!
12 690 Enduro-R, 12 Husaberg FE570, 09 KTM XC-F/ 450 RFS hybrid, 07 KTM 450 SMR, 09 BMW F650GS twin/F800GS hybrid, 03+05 YZF-R6