Juan
Car looks great. This is a tough question.
I would replace the rear bumper with a stock or better still a euro bumper.
Possibly add a P O R S C H E Stripe to the side.
Sell on BaT.
I think it should be worth minimally $50,000.
I would also consider reaching out to Paul @ AutoKennel
https://autokennel.com/
He does very well selling this type of car and is an expert of the market.
Here is a recent sale;
https://autokennel.com/featured_vehicle/1974-porsche-911-rs-sport-purpose-coupe/.
Luke