You will have to remove the exhaust elbow to check it out. They are replaced every few years or so. Unless you changed something I doubt it is your prop pitch unless you hit something with it. First thing I would do is check the prop. you may have snagged a rope or fishing line. It does not take long to have the prop fowled with barnacles this includes your bottom. When was it last cleaned. If it was not last week I would check that first before you start replacing parts.
That derelict boat was another dream for somebody else, don't let it be your nightmare and a waste of your life.