Between 2000 and 2002, Mercruiser had a precursor to the SC1000 Smart Craft system. It is close to being the same and during this period often newer attachments (gauges) came out that would work with both the SC1000 and the earlier version.
So the SC1000 version (79-889223K22 ) will replace your 79-859068-1 as long as the back of your existing guage is blue. If so then you can swap over to the SC1000 series.
However, if the back of your 79-859068-1 is white then you cannot replace it with the new quage. This means that your guage was one of the earlier versions and is not compatable with the later more developed systems. If it is white let me know and I will go see if I can dig up the original one for you. I might or might not be able to.