The M103 is pretty well tuned to begin with (for the 1980s). There's not that much that can be done to gain more than a handful of extra hp without spending $2k+.
Maybe you could fit a used rear diff with a shorter ratio to give a sportier feel at low speeds but I don't know how much the labour for this would cost.

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1989 300CE "Project HWA124"
(400rwhp Turbo Technics AMG C36 engine)