Their tractor page include this line: We are interested in a Piston Engine Construction Set – modular, in-line scalable engine that can be adapted to spark igni

Their tractor page include this line: We are interested in a Piston Engine Construction Set – modular, in-line scalable engine that can be adapted to spark ignition, compression ignition, or steam power. That kind of means petro, diesel, and coal. That's not a hard fuel requirement though.