There are a couple of ways you can do this. One way would be to get the tach that also reads engine hours from the engine. Specifically, to get the hours (and much more information) on your chartplotter you can create an on boat NMEA2000 network and link your chartplotter to the engine. The 50HP engine sends out a number of different types of data from engine speed to fuel rate to engine hours (see
https://cdn.powerequipment.honda.com/marine/pdf/misc/nmea2000_pgn_list.pdf).
Your chartplotter is already set up to connect. The connector is on the back next to the power connector and is labeled "NMEA 2000". All that is required to set up the network is to buy the required cables and connect them together. There is a special cable that is needed to connect to the Honda engine.
PM me for more details (I am going through the same issue with a Humminbird). I think I have found a source for the needed cables that should run not more than $75. Good luck!