(1) pull the dizzy and be sure the shaft turns. (2) Turn the oil pump, although if it is the culprit, removing the dizzy and drive gear from the cam shaft takes it out of the loop and if the engine will now turn over, the problem is a locked up oil pump. (3) Feel around the bottom of the cam chain cover for any dimple(s) that might indicate a broken chain jambed between the crank sprocket and the cover. A loose nut or bolt from the cam sprocket or chain tensioner could also be jammed in there too. Turning the engine backwards might free up the jamb enough to indicate that the problem lies in this area.
If you have no joy after these checks, pull the pan and look for a broken crankshaft or connecting rod.