Here's a
link to a US EPA pamphlet discussing emissions standards for small engines. It's geared toward retail dealers of lawnmowers etc. but contains a lot of information that applies to the consumer, particularly if you service your mower. Reader"s Digest Version: you can't modify these engines (under 25hp) or you'll face criminal sanctions.
One little bit of good news: it appears that pre-1997 small engines are "grandfathered" so you can modify those all you want.
The pamphlet doesn't talk about two-stroke engines (an interest of mine:smile

specifically but I have to guess that the two-strokes have suffered under these EPA regulations. I don't know of any new two-stroke mowers available for sale in the USA. However, almost all hand-held, gas-powered devices (chainsaws, trimmers, etc) are two-stroke -- since they're so light -- which is one of the things I like about them. So two-stroke engines can still be sold but, for some reason, they aren't available on mowers. Does anybody know more about this? I did some searching via Bing and couldn't find anything better than the pamphlet.
Anyway, I have a wonderful two-stroke Lawn Boy mower that runs great at age 30+ and I'm always in the market for another good used two-stroke mower.