The exhaust cover gaskets are available through marineengine.com
http://www.marineengine.com/parts/johnson-evinrude-parts.php?year=1958&hp=7.5&model=AD-12&manufacturer=Johnson§ion=Powerhead+Group
Finally, I am a bit concerned hearing you reusing the rings you had to more or less force off the pistons because they were stuck. I am not bothered by the fact that the rings were “used”, but am concerned that they were weakened/compromised by having to force/coerce/pry them off the pistons…
