It all comes down to you getting what you pay for - most of the time. Years ago the B&S and Kohler were fine. The same goes for just about everything in our country now. We are in a throw away society now where price drives everything including or disregarding quality.
As far as gas engines on mowers I've only had Kawisaki on my Deere zero turns with great success. Everything else I own is diesel including my wife's car. But now even a lot of the small diesel engines are suspect. I was looking at small generators recently and I automatically lean toward a diesel. Not only for longevity, but I always have a lot of off road/heating oil on hand. From the reviews I read a lot of the small single cylinder diesel engines on these generators are made in another country and are falling apart.
It's a shame your zero turn is turning out this way as you said it is a great mower. After running a Deere 757 for many years, when I look at the zero turns at the big box stores, I can see the lack of quality with just walking past them.
Since you are invested in your current mower already, you'll probably have to deal with it and keep if working. The only real alternative would be a $10K zero turn like I had. It takes an awful lot of mowing to justify that price, but so nice knowing with basic maintenance it will just keep going and going.
I had to give up the commercial mowing due to health issues, and had to sell my 757 and bought a 62D deck for my Deere 2520 for mowing my own 3 acres. Oh how I miss that 757!