The canteen and cap were in good shape but the cork gasket was missing. So, I went to Lowe's in search of a replacement. I'd read of using a silicone end cap for for pipes as a source for gasket material, but couldn't find any in stock. Naturally, the store staff was nowhere to be found.
I did find over in the adhesive section some DAP food/aquarium grade sealant/adhesive. (The link is to Grainger.) I put a layer a couple millimeters thick inside the top of the cap, being careful to keep it off the threads. Then I let the goop cure for a couple of days.
Tonight I filled the canteen with water, screwed on the cap, and shook it vigorously. It didn't leak, so I'm going to chalk up this experiment as a success.
One mod I may make is to attach the cap with a snap link to the body of the canteen. That will allow me to remove the plastic cap and place the canteen over a fire.
A cheaper alternative to the USGI aluminum canteens are the surplus French aluminum canteens