Looking at the oil pump 'access plate', you will see it is retained by 3 studs and nuts. When the 3 nuts are removed, the plate (which closes the pump body) will come away (with a thin paper gasket). You will then be able to see the pump shaft and 2 vanes with a light spring between them. Put the vanes and spring in a safe place before attempting further dismantling. You will now be able to see the flange on the pump body - there is a thin cork gasket under the flange. Carefully use a pair of pipe pliers to wriggle the pump on the studs until the gasket breaks free. You should then be able to withdraw the pump. (Don't try to lever the pump off the gasket as this can easily damage the flange). Withdrawing the pump is a simple remove and replace operation - there is nothing inside that can come adrift. When replacing the pump, fit the cork gasket between the pump body and crankcase, and the thin paper gasket between the body and cover plate (Don't be tempted to fit a thicker paper gasket as this governs the internal clearance in the pump). Put just a smear of grease on the new gaskets (jointing compound usually sets and makes future pump removal more difficult). With the pump removed, you will find it has a gauze screen around it to prevent the intake of particles large enough to cause blockages - these days it would hardly be considered to be a 'filter'! Being nearly 70 years old, these gauzes can be in a poor state of repair, but replacement of the mesh is a fairly straight forward soldering job.
Hope this helps,
Regards,
Eddie.