I have three gas cans, with three different spouts ranging from not too bad to is-this-a-psychotic-episode? I'm sure others have come up with this, but I did hatch this idea my own self: a tire valve stem employed as a vent. I had seen a stem installer like this somewhere along the line:
(https://i.postimg.cc/WzQTdpQv/IMG-3113.jpg)
I bought one on eBay (a story of its own) and drilled a 7/16" hole about where a vent would have been in 1975 and pushed the tool in, through the handle and to the spout opening. I screwed on a valve stem and pulled it through; things got a little bound up at this point, and I had to make a tool from a cheap old screwdriver.
(https://i.postimg.cc/Y0JMv2J7/IMG-3114.jpg)
A little gentle persuasion got everything where it needed to be, and a good yank seated the stem (now known as a VENT).
(https://i.postimg.cc/nr6pXV6t/IMG-3112.jpg)
I removed the core from the stem and put a cap on it; I'll let you know how it pours when I get the the gas station (loan approval pending).