In the old " universal magneto " the right hand breaker points were for the top.------Now the different style magneto found on a 1976 model 15 HP may well be opposite of that !!!
Take the lower unit apart.-----Inspect all pieces .----Inspect housings that hold / locate bearings.----Price parts / availability.------Perhaps start a search for a used lower unit.
Answer these.----What is your motor doing / not doing?----Do you have spark that jumps a gap of 5/16" , yes or no ?-----If you suspect 180 degrees out , then test by switching the wires.
????------This is a V-6 outboard powerhead.----Mounted as an inboard motor.------Mounted on top of a jet drive pump !!!-----It can not be tilted up and turned to fiddle with power tune or other potions !!
In the big picture a head gasket is cheap.-------Pull the head and have a look in there.----These engines live a tough life in the hands of joy riders.
When these engines left the factory there was a disc on top of the block with a serial #.-----Powerhead may have been swapped at some time.----Not going to happen , but if you walked over I could show you one of these 175 big bore crossflow engines !----Good luck with it.