Wow. That looks super clean
There's good value in keeping these as standard as possible these days. It's impossible to keep it bone stock, but keeping upgrades sympathetic will see you have a decent long term investment there. Whatever you do, don't start skidding it please
There's so few of them in good condition.
As for running rich - what makes you think it is? Common faults will be a boost leak somewhere - most notably, if it's still stock, would be the wing mounted intercooler - but it could just as well be as simple as a lambda sensor issue given the age of the car.
Good luck with it - they're a beautiful car and a very rare sight these days.