One thing i noticed when i did mine after having the tank replaced and then painted, was theres 2 coats primer, top coat then lacquer which makes the tank cap and especially the lock pivot, very tight, i cleaned mine around the pivots and added a touch of silicone grease to prevent rust and allow smooth operation, when i removed it, there was no spring so guess that one went the same way as yours into garage black hole!. It still works without just means you have to move the latch to lock it closed with your fingers rather the spring loaded. But the cap seal is spring loaded so locks it in place.
I'm curious how a pen spring would work in place as the original is like a clothes peg spring with 2 legs on it and roll pin goes through the middle, whereas a pen spring is just a straight upright spring?