There are a couple of surplus/trailer junk yards in So Cal.
The J rail is available at most RV shops today. Shipping can be expensive if longer than 8 feet in length. We have yet to replace any J rail. It's easy to straighten and looks nice with a sanding. Can be painted and/or polished.
Tear drop side markers are available at Vintage Trailer Supply and RV shops carry the more modern oval lights.
The diamond back splash was aluminum. It is available today at Interstate Metals in Milwaukee, OR. Request Mill finish; you have to buy a full 4 x 8 sheet and pay shipping. Stainless will work as well but is a bit harder to cut. Most aluminum will polish up fine and doesn't need to be replaced.
Here is some original on the side of our Little Dipper:
Grinder, there are some aristocrat Lo liner folks in this forum and it is great, I being one, and am new to this, so I will be watching for your work. I am in oklahoma, so eyeballin is out of the question. I am anxious also to find salvage in my area, so if I find any will give a holler.