This style accelerator pump uses a checkball and retaining screw which is accessible from the top of the pump housing. The checkball is 3/16” aluminum and is available here

The retaining screw is not available aftermarket, be careful not to loose or damage it.

If this checkball is missing, you’ll get a weak pump shot as fuel is able to pass back and forth between the pump housing and the bowl rather than being forced into the pump circuit.