Scaling up the manufacturing process.
You can make a heat engine like this: make a disc on an axle and around the periphery place little pieces of cerium, then bring up a magnet (which attracts the nearest piece of cerium) and place a heat source under the disc on the same side as the magnet.
The cerium heats up above the Curie point, ceases to be magnetic, and the next piece is attracted causing a slow rotation of the disc.
You could build a cerium magnet heat engine of several thousand horsepower if you could only scale up the manufacturing process enough. It is however unlikely to replace existing heat engines. Ever.