Starting Gas Walk-Behind Lawn Mower

Too early. Mayo advises unrestricted physical activity after 3 months. Up to you. My suggestion would be to pay someone to mow the grass until the 3 months pass. Why risk a severe complication?

Thanks for the info on the Mayo recommendation. Yes, I'm currently paying someone. I'm wondering, though, whether it will ever be safe to pull the cord to start a traditional mower, or whether it would be wiser to buy a push-button electric that is self-propelled.

Once you are past 3 months and fully recovered using a traditional mower should be fine. Within reason. But there is also nothing wrong with making it easy for yourself.

You can train yourself to perform a kegel prior to any exertion like this. I had PT after my RARP and I was taught to kegel prior to a sneeze, cough, laugh, lift, really any exertion. You have to consciously make the effort to kegel for about two weeks. After that it becomes rote. However, I wouldn't try much exertion for 6-8 weeks while you heal. I was given the green light to start riding my bicycle after six weeks and it proved to be too early and I had to wait another 5 weeks. Everyone is different.

Kegels are for neobladder. Not for urostomy.

I've had a similar concern about doing crunches and other abdominal exercises after my cystectomy. I used to have great abs and a flat stomach. My surgeon doesn't think there's a time after which we're safe from hernias, and suggested I use a hernia belt, like weightlifters use.
Kegels are helpful if you have pelvic floor issues, regardless of whether you have a urostomy or neobladder. But I don't think they prevent hernias.