MX bikes have always been modified for enduro competition, as have nearly all bikes used in some form of competition. Its not the type of bike thats modified, but how its modified that is the point here. The original IT 400 ( I WANT ONE ) only had 61 parts that were not YZ400 and it was used at the ISDT!
If a bike in question has been properly converted for enduro use,ie, lights;effective muffler;side or centre stands;etc, and meets other age criteria (Pre '85 ), I cant see why it can't be ridden in the appropriate event.
Internal modifications for horsepower, suspension or general performance would be allowed as long as these also meet period criteria.
For examples look at Rolf Witthoeft's Z751 ( a road bike ),all Bultaco enduro bikes were basically the previous years 'crosser,KTM enduros were the MC model with 6th gear unlocked and lights, etc. added
It's not the bike but the conversion that matters
Roger