The piston shows normal wear which should
Indicate the rest of the system is in good
condition. While this pump is adequate it is far
from robust. The blockage from the solder needs
to be addressed.
The mixture screw response you describe is normal.
Blockage in the water passages does occur but
I’ve never experienced that in any of the early
Johnson I have had. Blowing air through the
cylinder normally brings out some rust partials
but as you have already run water through
yours anything loose has likely been flushed
out. In any case don’t attempt to remove the
fittings from the cylinder as the tend to break
off.
Tubs.
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.