How can I increase the performance of my Audi 80's engine?

I have noticed a decrease in the performance of my Audi 80's engine and I'm looking for ways to improve it. What steps should I take?

First, make sure to regularly replace the air filter to improve air flow into the engine. Clean or replace the spark plugs as well, as they can become dirty and affect performance. Additionally, using a fuel additive can help clean the fuel system and improve engine performance.