Take the trailer to a lock smith, or if all of your hatches and doors have a common key, take an unlocked hatch off the trailer and carry it into a locksmith and have a key made. I did this, and I think I spent less than $20.
I don't blame you for wanting to replace the whole latch, but there is no drop-in replacement other than another very expensive KT latch. There are kits that essentially come with the latch you refer to, plus some aluminum patching material and some rivets that, you can buy, remove your old latch, patch the hole, and install the new latch. Takes a little effort, but certainly easier than reskinning your door inside and out, which is what I did.