I believe your trailer had a two-pane jalousie type window. If all the mechanisms are in place it would just be a matter of getting a couple panels of glass cut. If not, there are actually several places to get parts to rebuild the window to original or just install a fixed pane.
You may want to post your question to the Avion forum aviontrailers.net and maybe give Chuck Cayo a call, his grandfather started Avion and chuck is still in the biz repairing them and is a fount of knowledge. *+1 (269) 463-5068