Marine Tex is an epoxy that has been around for a long time. Epoxies usually work well if he leak is visible and the area is clean. I would also try one of the stop leaks before pursuing other more difficult solutions.
My first thought was that the leak was not coming from the engine at all, but from one of the hoses on the firewall and above the engine and then dripping down.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13