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.

As long as it includes ethanol, then it's sorta renewable.

If you have to you can run vehicles off of anything that burns. Thus the wood-fired trucks in North Korea.

Two acres of sunflower seeds will produce enough oil to run a tractor for one season.