The 'unaltered appearance' rule is almost impossible to enforce and needs to be reworded or better still, removed sooner than later. I've had DT2MX clutch and ignition covers on my pre 70 DT1 for years with no complaints, what about all of the aftermarket heads and clutch and ignition covers? What about the Wheelsmith cutaway fin conversion on Maicos, CCM fin conversions on BSA B50's & B44's...yada yada yada....the list goes on.
The rule was originally included to ensure that bikes wouldn't be radically changed so much as to lose their period identity but after a year or two it was realised that the paragraph wasn't needed. With the exception of my knocking back a '74 Mag Husky that had a 76 radial head fitted (more because the owner was a tosser who wanted to argue the point than because his bike offended me), I can't recall this rule spoiling too many racers fun over the previous 24 years.