I asked my insurer (AAMI) regarding a 20V Silver top in an AE82. After speaking to their engineering person, they said not a problem, and no extra to my premium.
Here in SA they said as long as it passes Regencey (the SA MOT place) then they would insure it.
I think the main reason they said yes was it was esentialy the same block, just a later model of the engine that runs on unleaded, they figured it wasnt a performance engine.
Worth asking, you may be presently surprised. If not, then you are at least still covered by the insurance.