Under the metal is often a flimsy 2" wood framing so that's the easy part.
If metal 'studs' cut to make them into a straight shape and double up with 3/4" plywood and again use screws to fasten the metal studs to the wood.
The corner metal you should be able to purchase locally from a trailer yard and if lucky even get a used section.
If you don't want to get involved with rivets you can always back any seams with wood and use screws and if you do so use strips of plywood for the screws to have a better grip.
Torn trailer 'skins' can be externally patched (not all that pretty, but functional) using pop rivets and sealant.
It can be done, takes time and patience.