Trying to get an old Seagull forty plus running. It has been stored for 10+ years since last run. Cleaned it up a bit but it wouldn't turn over. Could rock the flywheel a little but that was it.

Watched a bunch of videos and looked through SOS and suspected it was the gearbox. Separated the powerhead from the driveshaft and confirmed where the problem is. So now I am stuck, how to open the gearbox without destroying something?

Got the screws out Ok, looks like there is a gasket in there, but it's not showing any signs of moving yet.

You will destroy the gasket no matter what you do. I would tap a knife blade into the gap and soak some penetration fluid in the gap over a few days. Try something like acetone and power steering fluid 50/50. Just take your time.

Diesel is one of the best options for penetrating and freeing up the gears of an old seagull. I have fixed a few like that including seized pistons. Once cleaned up all that is needed is a clean spark plug with the correct gap and a fuel mix of 16 / 1. Hence the need to clean the spark plug regularly. I went all around French Island in my 15 ft clinker and seagull engine. Never missed a beat all the years I had it. I've had 3 seagulls and now all sold on.

Oxalic acid is good for freeing up rusted parts. Can be found as timber cleaner/brightener.

Got three of them
Two run, the other had a seized gearbox too,
Sat in diesel for probably 6 months,
I now have spare parts for the two that did run.

In the backyard.

Not game enough to actually try them on a boat.

Thanks for the guidance guys. Don't like the sound of the seized gearbox, suspiciously similar. Going to fill the gearbox with penetrating fluid and perhaps sit the whole thing in a bucket so the outside of the join gets a bit as well. Leave it like that for a week or so then have another crack.
appreciate all suggestions