Not sure if your interest is to monitor
fuel levels or to determine total fuel capacity. I am assuming that it is not the latter, but if it is, ask the PO what the volume is if possible or measure the tank dimensions (the latter may not be a trivial exercise because of non-square angles in the build).
Fuel gauges are typically quite simple and those that I am familiar with can be removed from the tank for repair and also provide access to the tank. Buying
a new gauge might be simpler than trying to fix a broken one if you have already looked at issues like continuity between the float sender and the gauge.
There add on gauge systems that are non-invasive. One of my boats has an Accugauge system that uses sensors on the outside of the tank. It is not super accurate but certainly provides indicative data regarding how much liquid remains in a tank.
In terms of monitoring fuel levels, using engine run time will work
. You may need to use estimates of how much fuel the engine uses per hour from the manufacturer and web sources. And then derive data for your boat from run times and how much fuel is added each time you fill up - noting how many hours the engine ran to use that much fuel. IMHO this is the most accurate system.
Good luck with this one.