I've never heard of anybody taking a hit for non-class legal control systems in a Portsmouth handicap race, but maybe the regattas I go to aren't that serious.

The downhaul and mainsheet upgrades may also be assumed in the main sail adjustment.

Just talk it over with the RC and then go out and have fun. The RC ought to be happy to have an extra participant, and will need to decide for themselves how to go about rating you.

If you've never raced before then I really doubt you'll be a threat to anybody who takes these things seriously, regardless of whether your D-PN is 70 or 60. If after a few races you start to get close to the serious racers, then re-evaluate.