in MY world, I would not recover the wood completely with the tin sheet.
Just remake your wood transom as you like and only use about an 8-12" strip
of metal across the top part. Whatever thickness you you can find. I use .080".
I am thinking the full tin skin on the inside was only cosmetics. Not structural.
Your wood is the structural part.
You mentioned you want to replace the entire skin . . .
I guess if by doing that, you are wanting to restore it back to original condition ?
If that is the case, can you really be sure it came from the factory with the full skin ?
Or, could it have been added on by one of the previous owners.
If you want to get a piece of aluminum sheet and replace the entire skin,
many sign shops use .040" aluminum sheet for signs. Most are already prepainted
with baked on enamels in a rainbow of colors. A few years ago, a 4' x 8' x .040" sheet
was $42.00. Maybe a sign shop could sell you half a sheet. Very easy to cut with a skilsaw.
Not much can take off the baked on enamel paint other than a random orbit sander
which would only take about 20-30 minutes.