Hopefully its not the gear housing gasket
What about the tappet cover weeping down to the area you describe, course the injection pump needs to come off to r&r this properly most times.
What about someone who timed it (lashed valves) who removed the timing pin and may/may not have changed the o-ring if it was bad? Or didn't reinstall the toothed washer that holds it in the timing pin housing
I've heard of a few degreasing then loosening the gear housing a bit and forcing some black RTV in their problem area, letting it cure then torquing the bolts back down. Of course how do you properly clean the gasket so the RTV is effective?
Injection pump to gear housing (o-ring) (unlikely).
What about the oil line to the vacuum pump, often the fitting in the v pump will loosen up causing it to weep. Loosen line, remove brass fitting clean it up and the vac. pump, use some teflon paste or a dab of RTV and snug it back up, not too tight or its heli-coil time. Support the fitting and snug the compression/flare nut back up. .
I wouldn't go to crazy inside the gear housing, what if you are crushing the gasket more and then it leaks worse?
Just my . 02
Andy