"22.2 Open roll cage style cars will be permitted to compete if the entrant can supply proof that the car is
closely based on a full size example."
All you need to do is fabricate a roll cage and you can totally ignore all the rules on ventilation.
http://www.brca.org/sites/default/files/library_files/BRCAchairman/Handbook2011%20Final%20with%20Mods.%20Publish%201% 2010off%20road.pdf