I have a Gen 1 STI 170mm that I just flat out dont use. Compared to the Gen 2 STI tubes and my MBX 170mm, I just dont see the point in keeping the Gen 1 "up to date" to compete with the other mags I have.
Does the Gen 1 mag work? Sure, but for the sake of simplicity in ordering/keeping around replacement springs and whatnot I would rather just stick with the Gen 2's/MBX. I would rather avoid the hassle of it possibly falling out in the middle of a match, and it needs a new spring so it just sits at home where I dont have to worry about it. Since you're already having sizing issues, I would just say get it replaced and call it good. Sell the Gen 1s to people who want/need them and call it a day, one less thing for you to worry about.