TLDR: examine the distributor (pickup, igniter/crank angle sensor, optical disc and sensor), power transistor, and ECM. For diagnostic procedures you'll need the FSM and some google-fu...
the black thing mounted on top of the bracket with 3 wires ( J121 Power Unit ).
That's the power transistor/ignition control module/power unit/coil igniter/igniter switch. The igniter inside the distributor is a different thing (confusing I know). Here's some diagnostic instructions from 2005 for ignition coil and power transistor, in case you haven't tried it this way yet: http://4x4wire.com/forums/ubbthreads.php/topics/608592/re-not-turning-over.html#Post608592
(later in that thread, 1/12k Ohm readings are said to be fine)
The 91's distributor includes a crank angle sensor/top dead center/camshaft position sensor, also called the igniter (this is the optical sensing unit, here's an article on 3sg explaining how it works: https://stealth316.3sg.org/2-cas-91-92.htm
); these are somewhat similar to the GM Optispark and Nissan optical disk sensor distributors of early 90s. If these start failing, it may cause weak spark/misfire/no-start (on complete failure) conditions. It seems possible to test it with just a test light at the sensor connector. The distributor pickup failing may also cause similar conditions. Looks like the part number for your distributor is MD148008 with the sensor part number MD618278 (seems discontinued and not available separately; found this via PartSouq and Ilcats, but Mitsu ASA or epc-data should have same info and nice parts diagrams). Here's some info on theory of operation (from a 89 Sigma/Galant w/ 6g72, should be similar to yours): https://workshop-manuals.com/mitsubishi/sigma/v6-2972cc_3.0l_sohc/powertrain_management/...
Some folks with Optisparks have had success taking them apart and cleaning the optical sensor; a common failure cause was water intrusion (e.g. from washing engine bay with hose). For the Nissan distributors, testing the optical sensor seems possible with a multimeter: https://www.underhoodservice.com/tech-tip-nissan-optical-distributor-test-2/
(and they posit that a failing PCV valve may coat the optical disc and cap terminals with oil, so a good cleaning may help).
I did see evidence of it arcing from the case to the steel laminates.
Does that suggest a grounding issue in the distributor? Is that one of the distributor igniter's functions?
It has had this short studder from time to time like it cuts out for a split second, just this time it stopped totally
The crank angle sensor or pickup tube would be plausible here, the power transistor also. Intermittent power transistor degradation could be due to overheating, once cooled down it may work fine. Final no-start could be due to power transistor failure. Additional data on when the stutters occurred (after long drives in hot climates? in heavy rain?) may help inference here. Also, the ECM should set codes for crank angle sensor and TDC sensor in certain situations, did you check it (maybe the dash bulb is burnt out and a code is present but hidden)?
The condensers/capacitors can be shorted also, but I'm not sure how that'd cause the observed behavior.
Just a spitball here, but it could be the ECM itself (from a thread circa 2005: http://4x4wire.com/forums/ubbthreads.php/topics/646126.html
; btw electronics/computer repair shops exist that can repair ECMs cheaper than buying new):
The coil develops its high voltage by energizing the primary winding. The period of time this is energized is called coil saturation or dwell. This builds a magnetic field that when the circuit is opened the field collapses to the secondary winding which in this case provides high voltage to the distributor cap. Excessive resistance in the primary circuit can reduce the secondary voltage produced by the coil considerably. So a bad transistor(coil trigger) in the ECM could in fact allow a weak spark at the coil wire, but will not be able to bridge the rotor gap.
The usual recommendation is to smell the ECM (in glove box area for the 91) and look for a fish smell when cranking. A more involved approach is opening up the cover and looking for leaking capacitors, damaged traces, water damage, etc.
Conclusion: it seems reasonable to suspect the distributor igniter (as OldColt said, these seem to fail occasionally), the ECM (30 years old from the era of quaternary ammonium salt containing capacitors leaking 10+ years later, corroding traces and causing havoc), or the power transistor (could have become removed from the bracket acting as its heat sink and cooked itself to death). All three seem plausible to explain sporadic weak spark/misfires until a sudden no-start condition. More information on diagnostic process and exclusion of hypotheses would be necessary. (Or you could put spare parts in and go from there)
PS: this is assuming all other parts of no-start diagnosis (air, fuel, compression, timing) are good (no clogged fuel filter, no dying fuel pump, fuel in tank, no towel in intake, no melted pistons or chipped valves, no skipped timing teeth, no sheared woodruff keys, no discharged battery/non-charging alternator, etc.)