Does farm equipment need to pass environmental regulations? I didn't think it did.
If it does I doubt there will ever be a startup in this area.
A workaround may be selling it as a kit. If it's a kit and not intended for road use you could probably sell whatever you want, similar to selling radio kits that are illegal to assemble.
This doesn't mean it runs dirty, it just didn't pass inspection.
I'm not really a fan of skirting all EPA regulations, but passing them is a huge ordeal and probably not something a startup could do without lots of VC that would tell them to just emulate John Deere. It also covers exactly one engine setup.
I would start reading here:
https://www.epa.gov/regulations-emissions-vehicles-and-engin...
All of the small family owned operators simply went out of business, and the large corporate drillers have little interest in small rural markets.