Why cant you pour some stabil intothe tanks and get it into the lines after running the gas out?
Further get pure gas, no ethanol.
I used to use Stabil that seemed to work. A neighbor (and expert mechanic) suggested Star Tron by Starbrite so I’ve been using that now for years. That seems to work a little better based on how less difficult it is to start the mower every year. Just regular gas.