Ansa Supersprint makes tubular exhaust headers for the M102 motor, but they have a tendency to crack if there's excess vibration from the engine. Rather than changing the manifold, you might benefit from a low restriction cat/muffler.
As discussed before, the W201 and most Mercedes were never meant to be tire smoking, stop light GP cars. The W201 was designed and geared to be a long legged hwy cruiser, but with a shorter wheelbase that still retained larger chassis comfort. Short of forced air induction or engine swaps, there's not many cost effective ways of adding HP.
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