I must agree with my esteemed colleagues on this.
If you have lost both your Governor and Engine Tach after installing the RH Magneto then it sounds like the wiring hasn't been reconnected properly.
I would suggest first checking that the pins are in the connector plug housing properly and not been pushed out. Secondly you can pull the back cover off the magneto and make sure all the spade connectors are correctly positioned and if that doesn't work then something else in the system has probably been disturb.
I assume you have spoken to your maintenance organisation about this problem.