This part? - Looks like you would need at 1 special tool for that operation. Still not sure which part you are referring to.
Is it riveted.....or are there small screws on the outside of the receiver that you could remove? - sorry don't have an 870 to look at.
I'm sure it would be an easy fix for any of the local shops.....just $.....probably $60-$100 for a simple replacement.
I always like to fixem myself if I can - Cheaper ...PLUS ....I learn a hell of a lot doing it.