some might not work if you run in series, or, you need a special unit?, proceed carefully!
this one seems to be able to run up to 3 in series. https://relionbattery.com/products/lithium/rb20-x
not sure on any of these - maybe contact them?; https://www.lithiumion-batteries.com/ or https://www.powertechsystems.eu/home/products/12v-lithium-battery-pack-powerbrick/20ah-12v-lithium-ion-battery-pack-powerbrick/ or https://www.ebay.com/itm/12V-LiFePO4-20Ah-Lithium-Battery-BEST-PRICE-IN-USA-SLA-Replacement/264340876970?hash=item3d8bf1aeaa:g:uHwAAOSwGBxc7X3i or https://greenlifebattery.com/product/12v-20a-lithium-ion-battery/
these claim to be replacements (gel) for your mower; https://www.mightymaxbattery.com/p3517063/12v-18ah-gel-battery-for-black-decker-cmm1200-mower.html
I also found this!; http://www.mynissanleaf.com/viewtopic.php?t=8415 and http://markselectricmower.blogspot.com/2010/04/battery.html
ouch - bet you paid a lot back then, too bad you didn't get more life outta them. Still, not 'horrible' I guess.