Maybe a bit obvious, but check to see what classes are still running. I believe some rallies have banned >450cc bikes.
Nearest thing I've done is Vegas to Reno which is a 534 mile desert race. I did it on an XR650R and found the bike perfectly suitable, though not quite as competitive as the 450s. I've had my 650R in deep sand and it gets along ok, but is certainly more work than a lighter bike.
I think my inclination would be a 530 or whatever the largest displacement light-weight ktm is if the rules allow.