As Alan mentioned, these early EW gear oil pumps are scarce, as most 1925-26 models were swapped out for the later style. However if it has been working for you, I would preserve it as there are plenty of EWs running about with the later pump but not too many examples of the original design. It is nice to see a few going.
With a gear type pump, the oil is carried around the outside of the gears between the teeth cavities and the housing, so the condition of the teeth is not terribly critical as it only (!) provides the seal between the meshing teeth and prevents the oil returning between the opposed gears. Most of the leakage tends to be from axial wear, past the ends of the gear. If the bearings and journals are worn, then the housing could also be worn by the outer diameter of the gears.
Axial wear is quite easy to fix with these type. You can rub down the thickness of the housing to equal that of the gears. A thin paper gasket will then provide the requisite axial clearance. Before doing this, you will want to ascertain that the pockets in the housing are square/flat bottomed. Generally they do wear evenly, but it is a good idea to verify.
Excessive diametral wear will require a new housing. I am not sure what the hard and fast rules are, but I would think if you have more than 0.002 inch radial gap between the tip of the teeth and the housing, then you are going to have too much oil leaking back past the tips of the teeth.
You could have a new housing cast, but the shape is quite simple and could be machined from billet if you are not worried about it looking authentic. Also you can consider aluminum, as plenty of gear oil pumps do have aluminum housings (as did Douglas later on in the thirties). Choice of aluminum would depend on the journal bearings. Those are probably small and ought to be bronze for the load capacity. If an aluminum housing cannot be bushed, then you might just make the entire housing from bronze, just like Douglas did.
-Doug