well, for just 'LAPPING' I've done quite a few HPDE's and not one of them 'required' a roll cage. Check with NASAproracing.com and ChinMotorsports.com for events near you .. or just browse thru the back of a GrassrootsMotorsports magazine, track days for different regions are always listed.
If its 'door to door' racing, then a legal installed 'additional rollcage' is required. The specs should be spelled out somewhere on the SCCA website, regarding tubing size and mounting points. get one made by a race shop or buy one from a manufacturer like Kirk or Autopower
i don't know what you mean by 'the stock build rollcage'
good luck ..