I got a beartooth recoil pad / lop kit for my 870 express youth 20 gauge at gander mountain. The gun has a 13" lop and I have a 14"-ish lop. The kit I bought is a neoprene slip on pad with spacers of various sizes. My "new" shotgun now has a 14" lop. I can't wait for turkey season!
You can get the same kit at your local sporting goods store (Cabela's, GM, bass pro, etc) for less than $20.
I don't understand why anyone would look for an alternative to buying a new or new used gun?..
I was going to sell it and get a new one but the guy at gander mountain said I couldn't do much better than the 870. Anyways, my dad and his friend both chipped in and got me that shotgun when I passed hunter's ed when I was 10.
Yes, keep the 870 you have for sentimental value, first gun. Add a 12 ga or something for variety. Nothing wrong with a 20, maybe find a nice used auto-loader.
The first time I shot the 20 gauge was at a youth organized pheasant "hunt". Like I said, it was the first time I ever shot it, and they gave me a big pheasant load. I was terrified of my shotgun for a long time.