Carver cascade rapide leak

Hi guys the Carver Cascade Rapide in my Symphony stopped working last night while we are away for easter, green light as normal on control panel but no hot water, i took the cowl off and burner unit out thinking maybe 1a fuse had gone, it was intact, however I noticed a small amount of water in the control board housing, upon inspection it seems the cylinder is leaking slightly, probably around the large O ring, and this has damaged the circuit board. The instruction manual mentions a bypass tool i can use after draining the tank to allow me to still use the cold water system without the tank filling up, it goes into the drain plug and screws in pushing against the non return valve, I don’t appear to have this tool but have several spare non returns valves, does anyone know what this tool looks like? Or where i could get one so i can use the van while i get the hot water unit fixed or any other way of doing this? Would like to continue our trip if possible

Thanks

It looks like a 3/8" dowel about 3" long with a step down at one end about 1" long.
I believe it is inserted into the drain hole and held in place by the drain plug.