The drive gear is part of the cam. I've never heard of one failing on a 4K. Drive gear drives the distributor directly. Oil pump is driven by the distributor. Fuel pump is actuated by a lobe on the cam.
In fact, nothing really fails on a 4K. There is nothing simpler than a 4K.
If the engine was running fine and then started doing dumb shit again it is probably something simple.
You say the issue started after doing the exhaust. Maybe you didn't put the intake manifold or gasket back on properly and you've got a stupid vacuum leak somewhere.