If you are speaking of your 1984 300SD turbo 126 you should have a piston type vacuum pump.

I was able to remove the front cover on mine (the one where the disc valves are located) and reuse the gasket with careful use of silicon sealant.
Note I also snapped off 2 of the little screws. I had to use a propane torch and penatrating Oil to get the rest of the screws out without snaping them off. I also used an Hand Impact wrench on the screw heads so that the heads would not strip.

The O-rings under the Valves can be replaced if you get ones that are fat enough in cross section. Note the directions that the valves are installed vefore removing them.

Since you don't have a new piston seal and O-ring you are likely better off leaving it along. Stretch has some pics of a vacuum pump take apart.

The Vacuum Pump to Engine Block Gasket you could make from a roll of gasket material.
You could reuse a good old gasket if you use dome sealant.

I found a place that has the vacuum pump for $300 but the piston end kit is $170. The new vacuum pump also comes with a new check valve. The valve used to be $28 by itself but the price on the valve alone has nearly doubled.

Kit part number for piston end is 0005861723

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84 300D, 82 Volvo 244Gl Diesel