The distributor drive gear on the 124 cam is integral with the camshaft - not a likely point of failure. Neither the parts nor the service manual illustrate it, but the gear on the bottom of the distributor shaft is a replaceable part.
Remove the rocker cover, rotate the crankshaft and watch the rocker arms for movement. If the rocker arms are ALL doing what they're supposed to be doing, chances are that the pin in the distributor gear is sheared. If NONE of the rocker arms are doing their business you'll need to pull the timing cover and check the cam drive. If SOME, but NOT ALL, of the rocker arms are being actuated you're looking at a broken camshaft.
Any of that make sense?