When did the 600s get the rear disc (I know the 250 got rear in 1990, not sure if the 600 got one at the same time, or later)? Are those parts the same as pre-90 CR parts?
What about the forks - if the '92 600 first got cartridge forks in 90/91/92, then you'd want your forks to be the same as the 86~89 CR RWU cartridge forks.
The basic bike is right, you might just have to back-spec certain major components to ensure it is pre-90 legal.