Nothing needs to be changed, just apply the current MSNZ tyre regulations under Appendix Six, Historic Competition, Section Five - Schedule T&C 3.13 – Tires.

Only tyres suitable for legal road use in New Zealand, or period and
tread pattern correct, bias cross-ply tyres (refer note) may be used.
(ie. road tyres having 1.5mm minimum tread depth across 75% of
the width of the tyre and around the entire circumference). Aspect
ratio is restricted to 50% minimum. The use of semi-slick tyres with
only radial grooves is specifically prohibited.
Note: An example is Dunlop CR65 brand tyres

http://www.motorsport.org.nz/sites/d...%20T%26C_0.pdf

Dale M