Possibly loose/corroded start circuit connection(s). Starting with batt post ground, take apart each connection - clean to bright metal with sandpaper/emory cloth. Leave batt ground post unhooked. Continue cleaning process at each connection but do sanding/reassembly of that connection before moving to next connection to avoid routing foulup. Do include batt lead to chassis connection.Corrosion can be "non-visible" - so clean each mating surface at each connection. Re-asm batt ground post last thing before testing. Good luck.

In addition to the above, don't forget things like starter motor case to engine block, engine mounting bolts etc. They are all pat of the ground circuit.

That said, it IS a 30 year old starter motor.

Google-

695244a.pdf

for a service manual.

***"Once you place a marine bat in parallel with it's bat it starts and runs fine."***
Huh! Basically that is a jump start. And if the addition of another battery in parallel (be it marine or otherwise) produces a good spin of starter..........would it not stand to reason that the starter and cables are good?
The only part of the Murray wiring left to question seems to me would be the battery posts or the surfaces of the cables that connect to the Murray battery posts.
Now, if ADDING that second battery works........have you tried using only the second battery by itself without the Murray battery connected?