Nice Sidewinder! Looks like it's in great condition too. The Kohler K341 is a great little engine, and shouldn't give you any trouble.
That gas cap/gauge won't work with the Hustler/Sidewinder tank. For one thing, it's too long. Your tank is only about 7" deep (8" to the top of the filler neck). I bought one just like it but 2" shorter, and it was threaded for a threaded filler neck. You can get a NOS cap/gauge like the one you have from Richard Clark, but he sells them for $30 each. I put a sending unit from Summit racing in my tank, and used a GM gas guage from Equus. Total cost is around $50, but it goes well with my full set of gauges and dash panel.