These engines are not harmless. They do exhaust rather nasty particulates when operating on diesel. I appreciate the tempest in the teapot aroma of this stand but just look at the air and water where there is NO or weak Environmental Protection Agency like programs.
If everyone in the US was messing about with an old Lister[oid], then yes - you'd have a fairly major pollution problem.
10,000 of the things in the hands of amateurs isn't worth a fart in space.
I'm sure that, if there was ever a civil servant anywhere on the planet with more than 1 1/2 brain cells to rub together, [s ]he would come to the same conclusion and allow a limited number of annual imports, e.g. on a "not for resale" basis.
Even our godforsaken EU (crafters of some of the most stringent regulations in the solar system) recognised that one could import "an item for personal use" which didn't meet EU standards (for safety, emissions, or whatever). You just couldn't bring in a container full & sell them.