The easiest engine swap would be a 5.6 liter m117 from a 80s MB. Just use it's stock fuel and ignition systems.
Megasquirt would be a good option for the current engine, it would have more power and better economy. BUT hopefully you would be able to find someone who already has the files for the 4.5 liter m117, because programming it from scratch is a lot of work.

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Csaba
1972 280SEL 4.5, silver