Quite a few years ago, when my parents still had their hardware store and they had an FFL, I bought one of the 7.62 Ishapore "Jungle Carbines" made from cut down 2A and 2A1 rifles. Mine had the same issues you are having, not feeding from one side of the magazine. In my case, it was the left side.
About all you can do is to try and tweak the feed lips and see if that helps. Mine was doing the same thing you say yours is doing, the round isn't coming up quite far enough to feed reliably. I SLIGHTLY opened up the left feed lip and tested. It didn't take much movement, but I did get mine to feed reliably by SLIGHTLY bending the feed lip. Go SLOW and test. It probably wouldn't take much to bend the feed lip too far and screw everything up. The 7.62 magazines aren't real common and I can see a screwed up magazine completely ruining the gun if you couldn't get another. I honestly haven't looked to see if you can even buy new magazines if your original gets messed up or not. Either way, from what I have read, even if you do find an original magazine, it will still need tweaked to work since the magazine wasn't really designed to me removable in the sense that an AR magazine is. They were able to be removed for cleaning but were really meant to be fed with stripper clips and the magazine left in the gun.
As for the mag spring being weak, it has been so long since I even handled mine, let alone stripped or shot it, that I can't remember if the magazine has a leaf or coil spring. If it is a leaf spring, try a .303 SMLE spring. I honestly don't know if this would work or not, but the gun is based off of the SMLE rifle, maybe they used the same spring.
Good luck. When they work, they are fun rifles to play around with. When they don't, a lot of the parts are uncommon enough that you will probably have a VERY hard time finding them.
Bub75