[Yosemite Sam]

Welcome to OCC Bill,

Lots of good advice so far.

You may want to do a little poking around to see if there is something that may have caused the blade to break.

There are any number of things that could have caused this to happen. Your 125 should have a shroud around the fan, it may have some loose or missing screws, the rag joint or other parts on the drive shaft could be in bad shape causing the shaft to wobble and the fan could hit the shroud. Take you mower deck off and look around under the tractor and see if there is anything that looks questionable. While you're there check to see how many blades are left.

If the fan has never been replaced it should be metal, those fans are usually pretty tough, the plastic ones are pretty brittle from the get go.

As the other guys have said, one blade missing usually won't hurt anything, one blade missing on a metal fan may give you a little vibration.

Not many problems with a Cub that the guys here can't talk you through, taking it apart or putting it back together, the guys will help you in either direction.