Muncie 318 trany speedometer gear help!

[Megawatt]

Hi everyon this is my first post.
I have a 65 Chevy c10 pickup with 3 speed on the tree and wanted to know if someone could help me with a question.
My speedometer does not work. I have disconnected and check cable and it's ok
I removed driven gear and found the drive gear inside on tail shaft turns freely with my finger.
I have the long tail shaft non overdrive.
How do I go about to fix this plastic gear. Does the trany need to be removed from truck. Or canI remove the tail shaft
I am not sure what is allowing gear to turn freely. Some sites show a clip that goes with that gear or maybe the gear is broken.
Can't find any rebuild instructions online.
What does it take to repair this plastic speedometer drive gear on output shaft
Thanks, Robbie