STUCK seems to be a common affliction of the early Kiekhaefer made outboards...perhaps because those are the only ones owners will part with. I think your plan to use a skunk juice of your choice as a first measure is wise. No point in using a hammer where a feather will do. More vigorous measures as required IF required.
If rod cap screws and rod cap can be removed...sometimes possible sometimes not depending upon location of rod/angle and how firmly staked the screws are....
IF that is possible, piston/cylinder can be separated from crankcase assembly.
From there you can let it soak while attending to needs of rest of motor.
Thanks for the reply Louis. I always look forward to you views/input on these old Mercs. Not sure when I'll start work on it. Seems I have a number of motors needing attention and priorities always keep shuffeling around.
Kirk