hey Alfred
After your phone call I dug around and found that spare left side door latch we had talked about.
I doubt that it is in any better condition than your latch, there is lots of wear on working parts, most of the corners on the catching surfaces are rounded off.
I could send it to you if you would like to experiment with it, these latches are riveted together, not much you can do to take them apart and make repairs.
Al
you could try making a "S" bend in the rod to the inside handle to take out some of the slack and see if that helps, but there is also the possibility that that cold cause the door to pop open or jam the locking mechanism, but I would try it any way, you could always straighten it back out if causes problems.