JZ, welcome to the group.
Preservation or replacement is always a conundrum. If in good condition, I tend to favor preservation. But that can also be a give/take. I have a very original (whatever that really means) 280 SL. The interior is complete, and the only "improvement" I have done is to have the seat pads replaced. The seats are leather and were in very good shape, with no tears or rips. They have cleaned up nicely. There is some fading on the top of the door panels, but I accept that.
As with the firewall pad, if it is not flaking away, I would try to keep it. The new ones are much closer to the original style, and are a good upgrade if the old one is really shot.
As Dieter typed, post a few photos. We just love to see photos.
And the folks here indeed want to help, to see another 113 kept on the road.