I wouldn't say mine is perfectly legal, but I registered mine in MT as an '86 4Runner.
USFS is mostly concerned about having a registration of some sort.
Need a reg in my home state to be ok in CA when I come down for KOH--and I doubt CA/BLM are up on MT law enough to question me. I have a license plate, and my registration has both my '86 VIN and my plate # in case anyone needs to see it.
I don't drive it on the street, as it is not street legal...but could be pretty easily made street legal enough for MT.