You're correct, the fixed panes in the cockpit windows are no longer available. A piece of lexan is about the only fix. As for actually replacing it I'm 95% certain you're going to have to remove the whole cockpit window assembly to do the repair. The diagonal aluminum center bar is held in by screws top and bottom that are only accessible when the window frame assembly is removed from the coach.
Somewhere I should have pictures of what the assembly looks like removed and I might have some showing the screws in the aluminum bar. I'll do some digging tonight to see if I can locate them.
A year or so ago I parted out a 1986 345 and I could retire now if I had as many of those windows to sell as people have requested from me
I'm not sure what it takes to have windows made but it sure would be nice if one of the vendors took on that project. When you consider that for each Argosy/Classic motorhome that was built, the windshields and those two cockpit windows are the rarest windows of all. All of the other windows were at least provided in multiple quantities so that means in general there are more available as spares.
Good luck with your repair