On the mating surface, between the pump housing and the cover, there is a pear shaped recessed groove. There is a similarly shaped rubber seal which sits in the groove and forms a seal with the flat mating surface when the two are screwed together.

It is not a flat gasket you can cut out of a sheet. You could try to inject a bead of sealant in the groove and let it set up before mating the two parts. If there is a sealant that is stable in the presence of fuel.

I only found gasket repair kits for mechanical fuel injection on the Bosch Tradition website but you might try there.