I also own a 1966 Overlander. If I were in your shoes, I personally would do the following to create the least amount of damage, cost of repair, but most important "access to parts" that will need replaced by breaking into the trailer.
DO NOT BREAK WINDOWS!!! (very hard to find replacements)
DO MESS WITH DOOR!! (Again, hard to find parts and fix correctly)
Go in through the large "Solardome" vent in the living room area. You will need to do the following:
1. Drill out the four rivets holding the cover to the lifters, then remove the cover.
2. Cut the screen allowing access to the four turn-tabs that hold the screen in place, remove the screen panel.
3. Remove the two lifters (one at each end) providing you less things to get caught on climbing into the trailer.
4. Drop down into the trailer
From there, there's a push-tab that holds inside door-knob in place, two screws hold the lock in the door. Remove and send to Mountainblue for repair.
The only thing that will need to be replaced/fix is "four rivets, a piece of screening". If you happen to break the "Solardome" cover, they can be purchased new. https://vintagetrailersupply.com/ast...later-vts-279/
Hope this helps, Do Not Break Anything!!!