I have learned ( long ago ) that trouble shooting on outboards starts with a compression test.-----Post the numbers here.-----Walked away with a low hrs 1964 model 5.5 Johnson from an---- AOMCI----meet.-----Half a dozen folks looking at it basically declared it SCRAP because of low compression...
I doubt you will find any mixture adjustment screws on those carburetors--There are no mixture adjusting screws !!------Checked the flywheel key?----All wires from CDI box going to the correct coil ?------Checked the reed valves ?
Replacing the head gasket on a motor like that is a simple job.-----Just 22 bolts.----Refinish ( flatten ) the warped head.----Replace rubber grommets on the poppit valves.----But if it is a salt water motor it might be a challenge.