Here's an idea. Just an idea.
I considered this many moons ago, and not just for emissions reasons.
What about running the engine on propane at idle and low load ? Some kind of manual switching
between propane and gasoline. You would only carry a small propane tank, not big enough
to drive 300 miles (unless that's what you truly want).
Running on propane is virtually zero emissions.

Actually, my original idea was to run a Six-Pack deal, with the center "carb" running on propane.