Byron's correct, Scamp
can help you. They haven't changed the door lock assembly since the beginning. Since your trailer is nearly 30 years old, the hinges are probably worn out too. The hinge assembly (replacement) will run less than $30. They sell rebuild kits (about $5 each), but it would probably be better to just replace the hinge altogether than patch and repair.
The door on your trailer is repairable too. Since it's original and fits that PARTICULAR trailer, when it's time to fix it....give the members here an idea of what's wrong with it. You'd be surprised at the fixes possible. To purchase a new door from Scamp
is about $200 + S&H...the S&H is expensive because the door is heavy!