The distance between the two black linkages is not a good way to time the engine. Borrow a timing light to set the timing.
Here in the US auto parts stores will often loan tools like this.Perhaps there is a similar service offered in France.
You should verify TDC with # 1 piston at the top of travel by pulling the plug and using a screwdriver to feel the piston movement.
Then you can count the number of teeth on the flywheel and divide that into 360 to get the number of degrees per tooth to measure the places to mark the timing points.
Be sure to verify the TDC as sometimes a backfire will shear the key that attaches the flywheel to the crankshaft.
I am working on a motor currently that had this exact problem causing the whole boat to be abandoned as not worth fixing.
