I have a 1994 200hp Mercury 2.5 EFI XRi with oil injection removed that has around 800 hours on it. It would not rev up while under load so I took it in to have it diagnosed to determine the problem. Shop has checked out the entire electrical/engine control and has diagnosed a bad switchbox for the port bank of cylinders. None of the cylinders on the port side were firing and they swapped switchboxes and it replicated the problem on the starboard side. So I am confident that I need to replace the switchbox. So here is my question for those that have more experience with this then I do.

1. Should I purchase a factory Mercury switchbox or do the aftermarket ones work just as well?
2. Would you trust a used factory switchbox from a used parts dealer.
2. Is there anything that needs to be done when changing out the switchbox other then just removing the old one and wiring up the new one? I would prefer to complete the repair myself and I have lots of mechanical experience I just did not have the tools to diagnosis the problem.

This motor is in great condition and has always run fine (until now). Never been in salt water and is almost exclusively used on inland lakes (so I am not really worried about being stranded offshore with it).

Thanks for your help and input.