The spark from the horn button means you have a bad connection at the button. Disconnect the battery, pull the horn button off the wheel and the nut that holds the wheel. Clean the rust out of the spring plate. That is what comes off after the nut. Check that the little button has a spring and a conductor and that it is still good. Clean it all up put it back together and hook up the battery. It will most likely work just fine.

Steve H