Old dusty auto parts store might have a brass cap in their trays of old dusty brass caps.
Look here:
https://www.mscdirect.com/product/details/04254405
1/2 x M27 Pipe, 1,000 psi, Brass ISO Port Adapter
NPTF x Metric Adapter, Viton, 1/2-14" Thread
I am getting male and female confused. Could be why I have no children?
Might this pipe size also be used with hydraulics? There is a whole nother world of brass hydraulic fittings.
If you tell us year, make, model, and engine size of whatever has this water pump? With some searching of the internet ya might find someone else who is missing the same part and what solution they came up with.
Here is a list of every junk yard on planet earth and easy to search parts data base.
https://www.car-part.com/
Buy a M27 nut and have a welder weld a hex or round piece of metal to it.
Is this "bottom pipe that comes out of it" clean? If yes? Spray male threads with some thin oil. Fabricate a cap from epoxy putty. Let it dry. Remove and reinstall with thread sealer.
Some type of cork pounded into inside of pipe.
Rubber hose on outside of pipe fastened with hose clamp. Plug rubber hose with hose clamp and something round and plastic from plumbing supply place.
If ya like to do things the hard way? Draw the part with https://www.solidworks.com/ then print it. Expensive 3d printers can print with metal.
I go to sleep now. Sure that I will have more ideas in the morning.