Hey Gman its been a while.
I have not done it on the Marlin, but I did it on my Sailfish after about four years due to a defect in the material. I got a bunch of pink blotches all over the vinyl coverings due to a process error at the factory that made the fabric. GW sent me all new bolsters for free, but I had to install them.
As I recall there is a small philips head screw on the underside of each piece near the brass circular weep hole at each end. You have to look closely, but you will find it. Remove these. You should be able to push the bolsters up from the bottom or by putting pressure with your hands on the face and slide up. There are circular holes on the plastic back board that clips slide into (the clips are mounted to the inside of the gunnel and slide in the back board holes and end up inside the bolster between the back board and the padding.
The material is stapled to the back of the backboard with SS staples. If you are only replacing the covering, you need to stretch it over (keep any vertical lines true) and staple. If you are changing the padding too, remember to get a good closed cell foam for marine applications.
If I remember correctly the bolsters came from the Wise Company in Tenn. The newer stuff has the GW logo on it - not sure if they are still using Wise. If you want to get the exact material, maybe call GW Cust Service and see if they will point you to the manufacturer. You might be able to get the fabric or have the factory make complete new ones (fabric, foam and backing) and just install as I did. The only thing I found was that the holes were a little different, and I ended up having to move a couple of the clips to align with the hole placement. No big deal.
To install, simply align the holes moving from one end toward the other and then slide down the bolster so the clip slides inside of the backboard and set the screws near the weep holes.
Good luck and let us know how you make out.