Rare cars you dont see anymore,

[procta]

Yeah you need to have like 70% of the original running gear to get age related on kit cars.. Or new parts ive forgot now i did look into it with my kit car but its still unfinished haha. The westfield mega blade we had was age related (2006) but had a 919cc fireblade engine in which they stopped around 1997 so not sure how that worked.

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Along with freelander diff, and brakes etc lol

I think some kit car companies supply the vin and chassis. A again depending on what Kit you buy. As westfields, caterhams 7 can be built from scratch as you say, and all you do is drop what every engine and brakes. So I think those can get the latest car reg, that's going at the time. 
Mark evans built a cobra kit car, and used a sierra car as a donor car, I think all that he used was the brakes, and axles and a few other odds and sods from the sierra. I think that even got the registration of the year it was built or tested.