My take on the emissions requirements is that the emissions standards are aimed at the engine class (specifically, large, off road, spark ignition) as noted on the EPA sticker affixed to the engine.
I don't recall there being any correlation between the type of fuel, or the grade of fuel, resulting in different emission standards.
Or to put it another way, the engine must meet the assigned emission standards for the EPA definition of the engine, regardless of fuel type (gasoline, propane, natural gas) or fuel grade withing the fuel types.
Obviously, fuels which combust more readily and contain fewer "hard to burn" hydrocarbon constituent compounds will make it easier to meet the standards, but EPA does not offer different standards based on fuels for the same engine.
A dual fuel engine capable of running on gasoline or propane must meet the EPA standards while running on either fuel.