Curious about the exhaust header, is it a double tube exhaust? One side looks different than the other.
I use an analog multimeter set on buzzer mode and attach one lead to the points wire and the other to ground. The reduction in sound tells that the points have opened. That's the BTDC firing point.
If you don't know the spec, as mentioned by others, start out real retarded as that was the norm for those days. Is there a manual spark advance on the distributor? It may have had a bar mount control to advance it as the engine reved.
The easiest way in this case to set the timing is to use a wooden coffee stirrer with pencil marks. Put the stirrer in the plug hole and turn to TDC, turning in running rotation. Mark the stirrer at the top of the head with stirrer hitting piston top. You can feel it pretty close by turning back and forth a hair. Make a mark 1/8" above that line. Turn the flywheel backwards- BTDC until the top Mark is at the top of the head. Set your timing adjustment so that when you turn the flywheel to match the second line (BTDC) , the buzzer reduces in sound. Adjust the measurement as needed once it's running. If it doesnt start and rule out ignition, check the carb and see if the plug is wet. Yet some starting fluid that has top end lube in it, available at a marine supply store.