Ouch! I have a Mirage on my best bike. Sweet shifting derailleur!
I wonder if there is an exploded view of those derailleurs. That might tell you what is possible to replace. Now, finding that part might be a tougher challenge, I don't see a lot of those derailleurs
And now you know - you always check big-big before the bike hits the road. Doesn't work? Screw in the low gear limit screw until you cannot reach that cog (or cogs)..