CAUTION: Perform at your own risk! I haven't tried this.
I recall reading somewhere that the shaft will pull-out from the front if:
- You carefully remove the white plastic piece by bending up the tabs that hold it into place
- This will allow you to remove the bar that holds the brass colored clip that keeps the switch plunger from coming out
- Then I think you can pull the white switch plunger out from the front being careful not to lose the tiny springs or contacts
- You can then either replace only the plunger or repair the plunger and put it back together
Mini-tutorial: How to disassemble Marantz pushswitches for better cleaning/de-Oxiting
One way to strengthen the broken piece is to drill a small hole in both the switch plunger and the piece in the button and insert a small metal piece or thread a screw into it. The you you can glue it all together. This could help the repair keep longer.
Hope this helps and give you options to explore.