I've got the simpler (circular) 2100 version. The output cable broke at the sensor ( the very brittle potting compound used on the sender had ifound it's way into the cable due to capillary action).
So, I had to buy a new sender. At the same time, I bought a Navman 5600 plotter, which will accept the fuel flow data and combine it with the GPS data; mpg, range, gal per nautical mile etc. I bought them together.
So, a plotter is one option as a replacement for your display; however, the plug on the end of the cable for the "simple" version I have (2100) is not the same as that on the end of the cable for the "plotter" version ! (5 pin vs 8 pin) I don't know what the 3100 has. Both types of senders/ cables / plug have only 2 conductors. At one time, Navman used to sell an adapter like this one
http://www.ebay.com/itm/NAVMAN-Northsta ... 33806cec20.
Navman became "Northstar" ( in Europe at least), maybe there is a " Northstar" version of your gauge ( or adapter for a plotter)
edit; a new 3100S display ( newer version of the 3100, to overcome known issues with the earlier ones), on Ebay UK
http://www.ebay.co.uk/itm/Navman-3100s- ... 3f225e8633 Maybe the last new one on Earth !