I'm assuming you are referring to the standards in the USA. The rules allow you to repower with an engine that has the same emission standards as when your boat was originally built. So if your boat was built during T2 standards time period, you may repower with a Tier 2 engine (which is what I did). The engine (JD 4045 Tier 2) was ok to put in my boat as it was built in 2002. Could not have installed that engine in a 2015 build. These standards however, do not allow manufacturers to build new T2 engines.
Here is the EPA page for Tier implementation. Saw nothing exempting engines under 135kw.
EPA Tier information
I'm tired of fast moves, I've got a slow groove, on my mind.....
I want to spend some time, Not come and go in a heated rush.....
"Slow Hand" by The Pointer Sisters