[christofoo]

Maintenance is a really big deal; spark plugs, injectors, EGR, PCV, O2 sensor, don't let the cat get poisoned or plugged, don't destroy your compression...

As for mods, the only one that I've been able to come up with is a block heater. Hood insulation would probably help for shorter trips. The heated O2 sensor is also good, as Daox said, if the car is a candidate for the upgrade (and doesn't already have one).

In the end, the most cost-effective emission reduction mod might be a bicycle rack, or maybe electric moped. Stay out of the car for short errands.

EDIT: Also reel mower or electric mower vs ICE. (Anytime you see a small ICE, even a motorcycle, it is quite likely to be very disproportionate in terms of pollutions vs gas consumption. By very I mean like 10X or more.)

I wish that vacuum insulated cats were available for retrofit.

As for grid vs gasoline, I believe that anything that offsets ICE warmup is worth doing, since ICE emission is strongly biased toward warmup (first 2-5 minutes). I.e. the block heater and electric moped almost certainly make sense. For longer trips the trade may not be so clear, i.e. EVs. And remember that the grid will (probably) get cleaner while an ICE gets dirtier.

EDIT: I'm assuming you mean a coal-fired grid, like the one we have here.