You are talking about the rear coolant log.
It connects the bottom of the Thermostat housing to the rear of the engine.
Since it is no longer available from Kawasaki, you may need to source a used one like this
https://www.ebay.com/itm/400065220126It's the same part for all years as the engine block never changed.
You will need new O-rings and it is very important that the holes where it fits into the engine are VERY clean and lubed with silicone grease.
To do that you will almost certainly have to remove the carbs. I cleaned the holes out with a dremel with a fiber brush attachment.
Fit the Thermostat housing to the coolant log before you fit it to the engine, lube the o-rings with silicone grease and ease it in gently until the 2 bolt holes are flat and flush. Then fit the bolts. Then fit the thermostat and the thermostat cover.
If you want to test for leaks you can get cooling system pressure testers that pressurise the system.
This will show up any leaks before you refit the carbs.