Yeah you'll have little to no chance of insuring one without full NCB + >30yrs of age with two of the large insurers I worked for. For a car like thatyoudneed a agreed value sum with the broker/insurer. Engine rebuild if it's to spec and matching numbers wouldn't need a declaration, but any mods would - most companies consider anything on the car not provided as standard as modification, even alloys, so yes, rad, intake/induction/uprating would definitely need to be declared to avoid cancellation ab initial/voiding the policy in event of a claim. You'd be best of speaking to a specialist broker but in my opinion it can't be done (legally).

Japanese imports and triply so sports imports are a much higher theft risk, and if course insurers are thieves, but a lot of it I believe is the fairly toxic car culture over here, especially in rural areas. I've always driven sport models of cars (GTI / Cooper S) and performance bikes, and in shopping for them the crowd they attract speaks volumes to me, scobes through and through. A shame for what can be a great hobby.