Your RIB is rather small for a diesel, although I have seen one or two 6m diesel RIBs.
The only real option that I can think of would be the 4 cylinder Volvo Penta TAMD22SX which gives 105hp. There might be a small Mercruiser sterndrive too, but I haven't seen one.
Also don't underestimate the work involved in changing from outboard to inboard -- new transom, engine bearers (this may involve substantial structural work), engine box etc.
You may be better off selling your existing RIB as it is and buying a diesel boat.