LOL Bob typical kiwi statement!!!!
The thing is to read the rule book and see what you can do to stretch the definition and interpret them to what you want to do
The UK main rules where
1 wheel base plus minus 1 inch
2 engine has to remain at same end of the car as originally built
3 silhouette had to remain the same
4 car above axle line be the same
So as time went by the cars evolved from street cars modified to having wide arches big engines light weight panels rolling thru to semi space frame front rear ends to full space frames and then sports car chassis with glass fibre bodies fitted all of
this happened over a 5-10 year period
The Germans had Gp2 cars then Gp5 DRM Japan had similar concept as did the Swedes/Fins Ozzies and South Africans all with varying degrees of regulations on mods allowed and budgets plus any manufacturer input

In theory we could have bought a car from any of these nations to race in UK an they would have been eligible but how competitive remains another matter depending on the year
I would imagine budgets in NZ where restricted due to being small nation fewer people and high importation duties back then so you had to be self sufficient and build your own cars and motors.
The likes of you Mick Hill in UK were "pioneers" doing it first and others followed once they had either learnt the mechanical skills or had budgets to build cars to the next level of rules.