Some other issues. Depending on the type of valve stem seals it uses the seals could be shot and or there could be too much clearance in the valves. That allows exhaust gases to get into the crankcase past the valve stem.

If the fuel injection pump has pneumatic governor vacuum leaks cause it to have a higher fuel output and they smoke. Also, the diaphragm gets stiff and develops over time.

Concerning head gaskets try looking on US eBay or UK eBay. I believe the 621 at one time was used as a diesel generator engine for refer units by Thermoking.

Best to find a Mercedes parts site that has the mercedes part numbers and get that number and do a search with the part number.

If you did a compression test you should be able to tell if the head gasket is leaking combustion gasses into the crankcase. The other issues would be an exterior coolant leak from the head gasket or combustion gassed getting into the coolant.
If you are not having those you don't have a head gasket issue.

Another way is to get the piston up on top dead center on the compression stroke so that the valves are closed, and you apply compressed air to the cylinder, and you listen to where the air is hissing out of it. It can also show up a cylinder head gasket issue. It will cause bubbles in the coolant or air will hiss out of between the head and the block. However, remember your radiator cannot take the high air pressure so you either reduce the pressure or remove the radiator hoses from the radiator and plug the hoses off.

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84 300D, 82 Volvo 244Gl Diesel