Can not say how to get lower cowl off. Marvel Mystery Oil sometimes is just what the doctor ordered for stuck LITE outboards. Not always.
Stuck may have several causes and degrees. Sometimes it is the lower
unit that is the problem. Sometimes it is due to one or both cylinders having corrosion issue(s) If your 12 hp is a Mercury made
twin similar to KE7 thru KG4 series I can make a suggestion...see if if it if flywheel will rock even a little and check with something
inserted thru spark plug opening to detect even slight movement of
piston in cylinder. This (of course) requires access to and removal of spark plugs.
It is entirely possible that you 12 hp Wizard was made by another
supplier, but, basic evaluation is the same. My experience with stuck
motors is when problem involves one cylinder, motor is usually worth
the effort. When both cylinders are involved it may take a lot of t time and effort better spent elsesewhere. Every stuck motor is different and every one who works on them brings a different set o
of skills and methods. Results will vary.
Cover may has Duzus 1/4 turn fasteners. Or there may be a hidden fastener under grease and grime.