Now this is a situation where you should have a volt meter connected to the starting battery. Most older motors do not have regulators and a bad connection or bad battery can cause the voltage to go way up and blow your sonar. Been there, done that. A meter will serve two purposes. It will tell you if your motor is charging and it will tell you if the voltage is too high. Even if you have a higher amp stator and regulator, it will serve the same purpose. They really don't cost much and are very easy to hook up. Just my 2 cents worth.
Fair Winds and Following Seas
Bill H. PTC USN Ret
Chesapeake, Va