Need to replace gauges

[zack_13]

I have twin 1991 200 TXRP,TLRP
I need to change the 2 tachs (6Y5-83500-03-00).
Speedo (6Y5-83500-A0-00) still works. Can I change the Tachs and leave the speedo, or do I need to change all three?
I also have the 2 hour meters and 2 water pressure gauges.
I don't think I need to change these too, do I?
What new gauges can I use, and what harnesses do I need, if any?

Thanks

[boats.net]

You only have to replace the bad gauge as long as others are ok.