As all have mentioned, they are very well known to snap connecting rods, on the series 1 engines, not so much on series II or the magnums, as series II and magnums have full oil pressure lubrication to the crankshaft connecting rod journals.
Also no compression release as mentioned.
Take the spark plug out of the dead cylinder and look in the hole with a bright small diameter flashlight as it turns over, you should be able to just see the very top of the piston as it moves back/forth in the cylinder.
if it is not moving, you have a broken rod with other internal problems.
When they bust a rod they usually break a hole in the block and most times snap the camshaft.
in other words a catastrophic failiture.
if that is so your best bet is to transplant a bolt in a later Magnum engine.
Also as the magnums are getting older. the availibity is drying up so many ppl are upgrading to the newer OHV engines.
Let us know what you find.