I think for the most part if the part was an option, or is standard fit on later models then most insurers won't be to fussed, as it will be almost impossible to tell if was factory fit or not (and a lot of owners might not know if a car has for instance a rear mudguard fitted or not - and was it factory or dealer fit?).
Similarly if a dealer performs upgrades to a new car in the showroom that are available as a factory option, how would you know?
A mainstream manufacturer might well keep detailed records, Morgan however..
Personally if it is performance related I would declare it, and sometimes visually as they could argue it makes the car stand out more to thieves - but in the case of the 3 Wheeler keeping track of what stickers may or may not have been on the car from new won't be an easy job! A lot also depends on the insurer, mainstream insurers will be a lot less forgiving than a Morgan specific insurer such as Gott & Wyne.
For the record I always declare everything, best to ask and declare what you need to than be sorry later as most of the time the price is no different.
