The MSNZ regulations for Schedule K cars means that in a closed car it is mandatory to have a roll bar and optional to have a roll cage. However, they need to be built to the design and specifications contained in Schedule A which probably means they are more robust than in period. The only cars that don't need "modern" roll bars are the genuine pre 1960 single seaters that can have roll bars that equal or exceed the period specifications - if you look at some pre 1960 single seaters, their roll bars were only really decorative.