By throttle unit do you mean the control box or carb? If you need a new control box try ebay or craigslist or post a want ad here. I had to run/splice new wires into my 68 mercontrol box and lube the cables. You may need new cables if they are frozen in their housings, if you need new the length is critical, if you are changing the layout of the boat you may want to run them in a different path.
If you mean the carb usually just a disassembly and clean out will get you running, If she sat with fuel the jet(s) will be clogged, also look at the fuel pump, thin rubber membranes don't like to sit for years in gummy fuel.