i can't for the life of me picture it,but removal would be possible i guess if you can see it (just tap one side with a screwdriver and hammer) but refitting a new one will be impossible unless you can fit a suitable sized socket or something to bash it in square.....they are interference fit so you won't just wiggle it in with your finger,it needs to be pushed in squarely

Bar Seal is good too (looks like a small white tube of smarties,you just crumble it into your header tank)