Re the ROPS I wouldn't jeopardized because as you say ally is totally forbidden now. For all the safety details read appendix K from the FIA you'll find the relevant Article.
A bespoke ROPS is a good option under condition that you're talking to an official supplier.
Wasn't the plastic boot floor homologated?
Whatever the part you consider from period bear in mind that you must bring proofs of period and that any modification/adaptation must have been legally done.
