My take on it is that if the forks were indeed from an 84 KTM with the drum brake, then it would be legal. The disc caliper hanger and the actual year of the forks are the sticking point IMO, although I suppose you could have made the caliper hanger to suit the KTM forks in 1984.
Going on Bahnsy's rationale that the forks are legal, being of the same or similar basic design as '84, even though they are from '85 or '86, that would then make the twin shock KX500 in another thread EVO legal if it had a drum brake front. Then it would meet the basic no linkage, no disc brakes, no water cooling guide lines even though the engine is from a later model. After all it's just a basic old air cooled engine design which has existed forever. BUTwe all know what would happen if it were presented to race EVO, and would be deafened by the cry foul. I don't really care and believe the more bikes on the track, the better. Maybe there should classes to cater for these such bikes, without the BS, as long as they meet the basic criteria. EVO ultimate or pre '85 super or so on. Wouldn't we see some trick bikes then.
Cheers,
K