Just thinking, keep the insulation wrapped around the container with the replacement heater. I get 90 degrees when the container is outside in the 20's as long as it's protected well. See if you can find a chunk of something rotting on the side of the road or leave out a bit of raw hamburger or something at room temp until it is green and rotting. Toss that in. Possibly, the bacteria in the bucket are damaged from the heat if that is what it was. The crack will be ok. Everything should shrink back together when it dries. If you are unsure, take a Zip tie and just wrap that end tight until you are done.