Your best bet is to find an intact one from a donor machine but Candace is right. There's no guarantees it will stay intact. You're dealing with 40+ year old plastic. It's at the end of it's useful life. It's a common complaint with machines in that age group that have plastic gears or camstacks, not just Brother machines. I have 2 machines, a Singer Touch & Sew and a Riccar that have failed plastic gears.
If you choose to fill the gap, you might try drilling across the break and inserting a screw to help stabilize the plastic. Your stitch quality will be affected just as Candace said though. The spacing of the bumps that form the stitches won't be the same as when it was new.
Rodney