Give Chris at South Jetty Marine in Ventura a call 805-218-6614. He has a radon craft and is in the process of changing out a tank, and has done a few others recently. Best of luck, you got a killer hull!
I have the same boat. There is a tank directly aft of the cuddly under the the large square hatch. You will need to get the new one custom made. Lots of sources. Like everything boat related its expensive. Mine is around 85 gallons.