85 has the rear brake linked to the shock (BASS) - most riders disconnected them anyhow.
Barrel, head and power valve are different, but if my knowledge of the 84 vs 85 YZ125s is relevant, you could easily port the 84 to be like an 85. Base gaskets are the same part number.
Suspension linkage is the same.
Apart from the stickers, the cosmetics are the same - it would take a trained eye to spot a drum-braked YZ250N as being a 'faker' bike.