Re: 1967 Volvo Penta?
Model? MD6 and MD7 shop manuals and operators manuals can be found online. I can only suggest basic things that you probably already know. Replace anything replaceable. Drain all fluids and replace with new. This especially includes the fuel. Stuff can grow in diesel and diesels are finicky about fuel. Make sure the tank is clean and the fuel is fresh. The fuel system will need to be primed to remove air after filters replaced and fuel replaced. Other than that, details of engine and symptoms will probably be required to get better responses from the experts.