This last holiday weekend I needed to get towed in I have a 1985 Banher with OMC I/O. Engine is fine but no forward or reverse. It started with a repetitive thunking. Then the sound stopped and so did we. I live in the south bay, however I am willing to drive to have it done right. This has alwas been a turn key boat that seldom had any issues, and she is my baby.
The drive coupler is a series of "fingers" sticking out of the drive and the engine, these fingers mesh with each other to propel the boat. The fingers don't lock into one another which allow trim and tilt. The repetitive thunking that you heard was the fingers failing and now that the boat will not move forward the fingers are completely worn off.
John Young can repair your OMC drive and his phone number is 619-392-5979.