The easiest way to improve performance on almost any engine is to develop a more aggressive spark advance map. OE maps are "lazy", i.e. the spark advances slowly with increasing revs. This is to keep the engine out of detonation in worst cast conditions. Emission controlled engines are even more lazy to generate high EGT, which promotes oxidation reactions in the exhaust system.

I don't know the architecture of your spark advance system, but typically on pre-'80 engines the centrifugal advance is just a set of flyweights and springs inside the distributor, and installing lighter springs will bring full centrifugal advance in earlier.

On the M103 engine family there is a resistor that plugs into a wiring harness pigtail that determines the rate of advance with engine revs. Simply removing the resistor will provide the most aggressive map. Most of the benefit is in the lower rev range where you spend most of your time, and you can experiement around with different initial timing settings to increase the total WOT advance.

Start by finding the OE spec for your spark advance map, then use a dial back timing light to measure it on the car.

On my 190E 2.6 five-speed removing the resistor considerably increased low end torque. Prior to the change fifth was unuseable below 45 MPH. Now I can drop it into fifth at 35. Around town fuel mileage increased by well over ten percent, but highway mileage is about the same. IIRC total WOT advance is 32 at 3200, but it comes in quicker in the 1200-2500 range. Initial advance on M103s is not adjustabable.

If you search R16/1 under my handle there is a lot more information on the benefits of a more aggressive spark advance map.

Don't waste your money on air filters or other bolt-on aftermarket pieces that claim "more power". Most of it is just unsubstantiated marketing hype. Lower restriction exhaust systems can improve top end power assuming the OE system is restrcitive, but I think most OE MB exhaust systems are relatively low restriction, even though they are very quiet, and many aftermarket systems can be unpleasantly loud inside the car.

The mid-seventies saw some of the lowest power ratings of the post war era due to manufacturers struggling to meet rapidly tightening emission standards. The 1975 Corvette base 350 CID engine was strangled down to 165 net HP due to the use of a single restrictive catalytic converter. The same basic internal engine configuration in '74 was rated at 195 with the full dual exhaust system and no catalyst. That's a 15 percent loss due to increased exhaust parasitic pumping power. My 1975 Cosworth Vega was strangled down to 110 net HP due to both the restrictive catalyst and a poorly designed exhaust system. With a modern low restriction catalyst and better design exhaust system they make about 140 net with the meager 8.2:1 compression ratio on regular unleaded fuel. That's a 27 percent increase with a proper exhaust system.

The most effective way to increase top end power on most engines (assuming a low restriction exhaust system) is to massage the head ports for better flow efficiency, but that's a pretty big job. The best time to do this is when the engine needs an overhaul.