I'd imagine it depends what kind of emissions you're measuring? Are we talking air quality or climate change?

Two stroke engines are pretty terrible in terms of unburned hydrocarbons and are disgusting for local air quality, which is why I'm glad they're being phased out in many areas.

I'd expect these tractors with I6 diesel engines to run pretty efficiently. I'd bet that the CO2 emissions from tractors are tiny in comparison from the emissions from trucks, fertiliser, and transporting the food.

Lawnmowers are usually four-stroke, with two-stroke engines reserved for lighter tools like string trimmers and chainsaws.

I would still guess that lawnmowers produce more emissions overall, given that there are so many more mowers than tractors. But they get used less often than tractors, so who knows? Either way, I agree with your thinking process, that the most economical way to reduce overall emissions is to focus on what are actually producing the bulk of emissions.

I don't know how much better cars and trucks can get, and for mowers maybe electric is the answer. Mine is gas-powered, and I know it runs rich. I would love to come inside after mowing and not smell like fuel, so I'm in favor of better emissions controls on mowers.