My digital volt gauge arrived today. It works, but I haven't calibrated it against my digital multimeter (which is also not calibrated against anything, incidentally).
I think I'll install the gauge on the mower using a 12v socket & fuse I pulled from a junked car.
Then this "revival" project will be officially done.
I've been using the mower regularly, though haven't been monitoring battery voltage the last couple of times. It'll be nice to be able to monitor it to protect them from killer discharges.