Welcome to the "Cracked Clutch Release Arm Club", whose members are legion. If you have been into Model A's for any length of time, this incident will have most certainly visited you at least once. The clutch arms are approaching 100 years old and metal fatigue is rearing its ugly head. 25-30 years ago we rarely heard about such a failure. These days it doesn't even raise an eyebrow when it happens. Unless you are going for "show", replace the arm with this modern design that is not only more robust than the original design, its added length will somwhat make using the clutch pedal easier:
https://www.brattons.com/CLUTCH-RELE...uctinfo/11270/
Marshall