You can replace just the spring. I'd be worried about the rest of it though. A spring takes a really long time to wear out, so my thought is...what else needs replacing. Check to make sure the plunger moves freely inside the coil's sleeve at the very least. Make sure it doesn't have resistance. That would indicate a melted sleeve or debris in there preventing it from coming back down. I had to replace an entire coil because the sleeve was badly melted to the inside of the coil. The flipper would get stuck at the end of the sleeve causing it to not come back down. First 70% of the action was fine, but the last 30% was melted and caused the plunger to get wedged. I found that out by physically moving it and feeling the resistance. Check that to make sure it's not just a spring issue.

I would still recommend replacing the sleeve if you're going to be ordering parts. I have a Firepower and I've done some work to the flippers and pops before. It can be intimidating at first, but it's really not bad. Push yourself out of your comfort zone and you'll be glad later(I know I was).

Also! I had a weird issue with my Firepower where one of the flippers would stick up. I couldn't figure it out, but finally I saw the EOS switch was bent badly and the part that touches it was binding on the sharp edge of the EOS switch. Check that as well. Just a thought.