Fixing the leak in the exhaust did help a bit but wasn't enough. The manifold was also leaking and turned out to be bent a bit. After flattening it and remounting with a new gasket the engine runs like new! Solid and sound! Perhaps a new muffler one day and it'll be extra smooth.

I also renewed some fuel-lines, repaired a hole near the hydramatic, new coolant lines, refreshed the coolant itself, removed air in the radiator underneath the front seats, put on a selfmade gasket on the carburetor and removed some rust.
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Engine without manifold, carburetor and air filter. The black spot on the right is the middle of the manifold. The soot clearly shows a leak.
Part of the hydramatic. It's supposed to have a tiny hole (which is still there) but in my case it was damaged. To prevent dirt on the inside it's better to seal the damaged hole.

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