Yes, in fact I did just that this year (after owning the boat for 17). Got a Tempo mechanical fuel gauge from West Marine for $60. I needed one for a tank depth of 6.5", the shortest they had was 7.5.
For me it was pretty simple to install. I had a 4" diameter metal plate screwed on the top of the tank. All I had to do was remove it, and with a 1.5" hole saw, cut a hole for the gauge. Drilling the aluminum was easy. Drill and tap 5 screws for the gauge. No worries about contaminating the tank. I don't know if that plate is on all tanks that did not have fuel gauges from the factory.
