Ether in most kubota engines is a big no-no. It's a crutch for a worn engine or one that's glow plugs are not functioning properly. If someone's had to use either to get it started, you'll want to have the injectors checked and glow plug operation checked. If those check out fine, pull the cylinder head and oil pan, pop the pistons out and see how bad they are. Keep in mind that the smaller the engine, the more sensitive it is to scratches in the bores and ring wear. It needs to develop pressure to make enough heat to ignite the fuel. There simply isn't as much air in the chamber on a smaller engine as there is on a truck engine for instance, so any air that leaks past the rings, gaskets, injector seats, etc, is heat lost-making it harder to start.
Ether does a great job on them as far as breaking parts. Broken Rings, ring lands, pistons, piston pins, and also can (and has) bent connecting rods. Those are a few of the results of using ether on Kubota's that I've seen over the years.