I found this:
https://assets.publi...ge-guidance.pdf
Its interesting that it talks about the basic engine but not carburation etc. so I would take that as mods to the engine ancillaries are ok as long as they don't change its basic characteristic (i.e. its still basically an A series).
Interestingly it mentions that alterations to the type or methods of suspension constitutes a change, does a change from rubber to coil springs fall in this category?
What if you convert from hyrolastic to rubber? It's a change in the 'method' of suspension, while minis had both its not the Method That particular car left the factory with.
Now thats an interesting one, the guidance makes it clear that other engine options for the car are allowed as a swap (i.e. changing a 1000cc form a 1275cc or say a 1.3 4 cylinder capri to a 3.0 V6 engine, the suspension is clear that a change in the method of suspension from original is not allowed.
I reckon if you asked for clarification they would say the same as for the engines, i.e. if it was available on the car as original but not on that model year its allowable but thats not what the guidance says at present.
Its a veritable minefield thats clearly been dreamed up by someone who doesn't understand the classic car scene.
As another example, a lot of MK2 jags have a rack and pinion steering conversion, they have been around since the early 90's at least, according to the guidance thats not allowed and therefore the car needs an MOT.
A lot of this stuff will never get picked up, its just when something goes wrong and there is a massive accident and an inquiry where it gets picked up that they will clamp down.