Dont' know if this will help at all but here's a look inside my Mallory comp 140 I just rebuilt. I was able to re-use my gasket and o-rings. Seems to be pumping again. Need to test to see how much pressure though. (b4 and after pics)

Yours is a bit different in design. The plate covering the pump head must be hiding some type of gerotor assembly. It may have a gasket or o-ring seal...or like mine it could have both. I agree with Bill. No need to drain the tank. Pinch the hose off with either some welders locking pliers (type with the flat metal tabs)...a couple small pieces of NON sharp iron stock and vice grips or some wood scraps and a c clamp.

Second on the leak possibly being at the fittings. ORR....It might be leaking down the threads of the bolts that hold the cover plate to the pump head.

I've used teflon tape in the past but it's not suitable for all locations and a piece tearing during re-assembly could end up in your carburetor. I used Rectorseal 5 recently on the regulator set screw in my Mallory pump as visible in the photo.

I spoke to the REP at rectorseal a few days ago and he said Type 5 is resistant to natural gas and gasoline and can be put into service right away after assembly. You can get a small yellow tube of Rectorseal 5 at home depot. Make sure its says 5 on it and not just the regular rectorseal.

I'd remove the brass fittings and use it to coat the threads. Watch out for any old teflon tape in case there's some there. If there is carefully pick it out of the threads so it doesn't go through the fuel system. Same for the pump head cover bolts. Don't take the cover off unless you have too because it might not be a leaky gasket and you don't want to damage it if you don't have to. Remove one bolt at a time, coat it lightly then gently snug it until you get all of em done then alternate tightening corner to corner but don't overtighten so you don't damage the gasket or warp the cover.

Worst case scenario...you end up wasting a little time and it turns out there is a bad pump gasket or o-ring but go the conservative route first.

Last thought. If you still have a leak after applying thread sealer and have eliminated a gasket or o-ring as the source...or have replaced them...inspect carefully for cracks in the pump assembly. It is Chinese made! (not much that isn't)

Good luck.