I just finished replacing all the wood on the sides of the roof of my 1990 Coleman Chesapeake. It was not too hard to do. The outer skin is aluminum glued to foam and then a very thin two layer wood panel completes the construction on the inside.
Measure from the bottom of your ceiling rail, the piece of aluminum that covers where the ceiling meets the side wall, to the top of your canvas rail. You need to reset the rail to that same position when you reinstall it. Remove the canvas.
I drilled out all the rivets that went thru the old wood on the inside. I left the rivets into the ceiling intact since I was only replacing the sides and not the ceiling. These are special rivets. Called peel rivets or sometimes plywood rivets. Not to mention once drilled out they can not be replaced in the same spot unless you replace the wood they went into.
I worked on side at a time. The lower aluminum rail was easier to remove by using a universal painters tool, or putty knife, slipped between the outer skin and the rail. Work a couple inches at a time. If you get to aggressive you could cause damage to the roof skin. Once you get the knife in the entire length of the rail it will pull off rather cleanly. There were some screws from the outside too that needed to be removed.
Then I simply removed the rotton wood. Most of it fell out. Note that your canvas rails are screwed thru the wood and behind that wood is a metal strip about two inches tall. The screws go into that metal strip which give the screws substantial bite. If the metal strip falls out, you'll be able to glue it back in place to the foam. But do not omit it or your canvas rails will not hold! Out of the four sides of my top, I had to glue one side in. The others stayed adhered to the foam. Always pre-drill all screw holes and go thru this metal strip with your bit. I used a piece of automotive vacuum line over my 1/8 drill bit to prevent my inadvertent drilling thru the roof skin.
I used 1/4 luan. I measured the height and had Home Depot cut a whole sheet in strips to my measurement. I think it was 8.5". One sheet was enough for my 11' box. I then cut pieces to fit the length. Once all the cuts were made I painted all the pieces of luan with exterior paint for added protection from moisture. The luan should go under the roof rail on the inside all the way to the ceiling and then should be no lower than the bottom edge of the foam on the inside. You removed two cheesy layers of thin veneer and you're going back with 1/4. The bottom rail is set for that original thickness but you can get the rail over the luan. Could cause your vocabulary to slip a bit however.
Slip your nicely painted luan up under the roof rail on the inside. Now get some duct tape and hang the end of your lower rail just under the edge of the roof. I work alone and never heard the tape complain it was tired. Take the other end and using strong hands, putty knife and a rubber mallet and a cool drink and start slipping the outer skin and the luan into the rail. WATCH the outer skin and don't use the mallet until the skin is in the rail or you will bend the edge of the skin. I went about one or two inches at a time. Put a screw in the rail at the end when you get that end up so it won't slip off as you progress to the other end. (toward the tape) The rail is really tight but does go on. I used #10 x 1/2 screws where there were once rivets. Use the #10 x 3/4 screws that came out of the canvas rails back in the canvas rails. Don't forget you have to go thru that metal strip.
Make sure you caulk the upper lip of the rail on the outside to the roof metal. I only use long life 50 year paintable sealant. It wipes off while still wet. Silicone makes a mess and if you ever want to paint your roof you will regret regular silicone!
Took me a weekend but she's back together and really solid. My ceiling was mostly solid but a bit of rot where the boat rack was improperly mounted looked ugly. I used 1/2 Styrofoam which is pure white and secured it using Xmas tree retainers used in autobody repair. I just drilled 1/4 holes thru the foam and into the ceiling then pushed the retainers in place. I sell automotive hardware and the vendor, Auveco number is 11146 or 15414. It's a very light weight ceiling repair. Any other cover I could think of would be too much weight or might sag. Remember that inner "wood" panel is paper thin wood veneer and won't hold much. I got a little extra insulation too. Note: I did remove the door rails from the ceiling rather than work around them. I simply lay the door on the bed when I take her down.
Hope this helps. Good luck.