You don't need an exact match, you just need something with a profile that fits into the opening for a close match in size.
You also don't need a gasket channel. There are a lot of gaskets which will work very well for you that come with adhesives on the surface that mounts to the frame. You just peel off the protective paper liner layer and stick them in place.
While once in a while you can find an exact match for vintage parts from places such as Vintage Trailer Supply many times you won't be able to find one. But that does not mean you are at a standstill on the project. It just means you need to analyze the shapes in the area you are working, take measurements and find something with the basic overall profile shape. Remember you don't actually need those gasket channels so that makes your search for an extrusion much more likely to return a suitable substitute.
It might also be that you can do some mending on yours to fix a damaged section. There is aluminum soldering and local welding shops will sometimes do that work. Or you can even attempt it yourself. But don't try it on your irreplaceable part until after you practice on some scrap material. See this link for the rods and the repair methods.
https://www.alumiweld.com/
You could possibly create a backer piece that you adhere to the original if you are missing some areas. Sheet aluminum is not difficult to obtain. You can anneal it with a torch to soften if needed so that it will form more easily. Just read up on annealing aluminum as the color change and temperature needed is different than for other metals.