I would start with the PCA Spec Boxster (SPB) rules.
https://www.pca.org/rules-licensing-forms
BSX is a subset of the SPB modifications, designed for DE and TT events, while still being streetable. BSX is merely a suggestion made by the socal Porsche club, and not a recognized class. But it's a good place to start on your way to full SPB.
https://porscheclubracing.org/technical/general-competition-rules-gcrs/
Here's a spreadsheet I created with build parts and costs for SPB.
https://docs.google.com/spreadsheets/d/1bxSiHAHlNm6Bpi8IDsrKyRIvAFFqzrNUFzGQLTWr0NU/edit?usp=sharing
I personally think those stock roll hoops are useless. If I'm on the track, I want a full cage, including the FIA bars that support the front of the cage.
Hope this helps,
Greg