Well, a broken or sheared flywheel key WILL NOT cause a loss of compression, no matter which model or brand of engine is involved.
It is also very unlikely a blown head gasket would suddenly cause a complete loss of compression. There would be more obvious symptoms such as profuse smoking out the exhaust if a head gasket blows on an Overhead valve engine.
And that right there is where your description of the engine stops.
We do not know if this is an OHV engine or an L-head engine simply by stating "14.5 HP Briggs".
Look on the engine for some identifying numbers and post them so we can go further. The critical numbers are Model number and Type number, and they will be listed just like that somewhere on the engine.

Given your description, it is possible that a big chunk of head gasket blew causing at least a no compression feeling . I had one a short time ago that blew about an inch out and made only 25 on the gauge. More commonly, the head has let a valve seat slip out of place and it is holding a valve open.
Do this. Remove the spark plug, take something similar to a small wood dowel and turn the engine screen on top by hand to see if the piston is travelling. If it is, off the rocker cover comes and if there is nothing visibly wrong there- off the head will need to come. As a hint, with both push rods removed, the ends of the valve stems will protrude from the head relatively equal. If not, something is holding a valve open such as a seat that has dislodged or a screw, piece of debris etc. Using that same logic, it is a good time to verify that both guides are protruding about the same distance from the head as well.