https://www.catalinadirect.com/shop-by-boat/catalina-36/engine/instrumen...

this the sending unit I use. I have 25 gallon tank. I believe it was 11 " long. You have to get the right length for your tank. It has Reed relays inside its rod and a floating donut shape magnet floating up and down triggering the relays and indicating the amount fuel. 
the screw pattern will match your tank screw pattern.
PM me if you need more info.

My sending unit works, but its leaking around the gasket.  I was looking for some videos on YouTube and saw some good ones that show how to test so you know if it's the gauge or sending unit. 

https://www.homedepot.com/p/RectorSeal-4-oz-No-5-Pipe-Thread-Sealant-256...

This is what I use to seal the fuel tank sender unit plate.
Apply the yellow pase to both surfaces, then wait for 10 minutes then assemble. This paste is suitable for drinking water aswell.
 

Rich, thought I'd replied to the fuel gauge issue some time ago. Maybe on another post.  My gauge was off and I fixed it by opening the round 'lid' with the ~8 machine screws and pulling out the float arm assembly, then bending it enough to ensure that the float was no longer binding on the pickup tube.  After some fiddling to get it to read at the correct level it has been fine for a couple of years.  Had to add a fuel-resistant gasket goop under the round plate, or you could cut out a red rubber gasket or even use a red rubber canning jar gasket that fits and had the screw holes drilled into it.