My friend gave me his old 8100 because he switched providers. The phone is a little beat up and I was just wondering if the OEM faceplate and battery cover for the 8120 or 8130 will fit on a 8100?

No don't think it will, just a little different.

I have an OEM battery and cover for my 8100 I'll sell you for $15 plus shipping from AZ. It's like new, I used a case, and the battery I only used a few months.