There are going to be a lot of gray areas for sure.
Subtractive mods should always be ok? porting, drilling, hacking, etc.
Replacing non performance consumable parts: As long as its not improving it over NOS, should be ok. Tires, cables, pads, plugs, damper, etc. if your not sure send one of us a message.
Additive mods are tricky but error on the side of caution. You can make an arrangement that cutting, flipping and welding the baffle of a second "stock" pipe to make an chamber is within the rules... but might be pushing the spirit of the challenge?