You can source O-rings in a old style auto parts stove, will cost though. Send MSR an email and ask if the newer kits will work. The old style fuel line may be an issue, I changed mine out years ago for the newer style fuel line. I have the original Int'l style from '89. Sounds more like an issue with the bigger O-ring at the top of the threads on the pump. If you look close, you should see cracks. The MSR stoves will work for years, easily maintained. The old black and grey pumps are fine. The collar that holds the pump plunger in, can unscrew a little, causing its halves to fall out and you into a near panic if in the dark or over an old fire pit if that happens. There is a small O-ring inside the bottom end of the pump, it could get hard and not allow the ball in there to not make a proper seal, allowing fuel to escape, not good on a gas stove. Once again, they are easy to work on, I have over 2 dozen old MSR stoves, going back to the original model 9. The worse MSR pump is the red/blue version that came with the early DragonFlies.
Duane