The speed of the motor is controlled by advancing or
retarding the timing so there is no timing adjustment.
The timing is changing as you twist the speed lever.
New compression was likely 75lbs. I don’t have
anything giving that specification but that was
common for motors at that time because of the
low octane fuel.
I have a 1936 – MB-20 Twin, and a 1937 – 571-20 Twin.
This is the compression readings for them.
I suspect your readings will be similar if it was run much
as the oil back then wasn’t what it is today.
Do you have spark?
Coils are often bad in these and hard to come by.
Tubs
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.