My 1959 12 hp Elgin has 85 & 86 psi compression, and runs good.
My coils were okay, but if your coil laminates are the same size
as an OMC universal coil, it’s usually an easy job, but sometimes
you have to get creative with the plug wire routing, and looking
out for interference with the rotating parts.
I’m not seeing what impeller I used on my 12 hp, but also not sure
if the 59 is the same as your older unit, but I believe it was a simple
swap for mine.
I’m not sure I even had my tiller handle off, but perhaps you can
post some photos of your motor.
Here’s a photo of my tiller handle….
This manual may help some overall on your journey.
1956-63-West-Bend-12-thru-20-hp-models
Prepare to be boarded!