I could be way of bases here and I hope someone else chimes in here, but I don't think you have a big issue. Assuming your ECU is working, which may be a big assumption, I think you can simple "plug in" your RPM sensor into the ECU and you should have your tach back. You may need to splice it in, but if you reference the tech manual 14-50, and 14-100 you will find the schematics outlining the ECU and the RPM.

Again I believe the tach amp to be in the same as earlier versions it's just built into the ECU, so I think it should work.

Has anyone ever tested the ECU? I spoke to the dealer, and an Independent and both said that they don’t test them; they either leave them broken or replace them.