I don't think there are any numbers, or at least not official ones. (Disclaimer: there are no official numbers that I have ever seen and understood that were applicable). We could do a little number plugging with what we can find, but the problem with environmental problems is that there are so many variables (many of which are hard to place a numerical value on) that it's difficult to come up with a good estimate.
What did the article say, exactly? That per mile driven motorcycles (2-stroke, I'm assuming) produce more emissions than an SUV? I know there are some people on here who will know what those harmful emissions are, and they might even know numbers or percentages that each environmental offender (motorcycle and SUV) emits. I'm guessing that this is what the article you read was talking about, but I'd be very interested in seeing the article if you can link it or quote from it.
However, you also have to take into account that mopeds use less fuel (less pollution and impact on the environment from when it's pumped from the ground to when it's pumped to your vehicle) and that, at least for vintage mopeds or motorcycles, we are essentially reusing something that many people would throw away.
There are essentially two ways that people here use mopeds -- either as legitimate transportation, or as a hobby. Most people probably use it as both.
If you're using a vintage moped for transportation, you're reducing gas usage and you're reusing something that would otherwise be taking up space in a landfill. So those people don't count.
If you're using a new moped for transportation, you're riding something that complies with EPA standards, so you shouldn't be producing that much pollution. So those people don't count either.
If you're using a moped for a hobby, well, everyone is allowed to have hobbies, and all the hippy tree-hugger environmentalists need to bugger off. So those people don't count either.
Then again, I'm biased, so I don't know how much my opinion is worth.
"Mopeds and environmentalists":https://www.mopedarmy.com/forums/discuss/1/311588/311588/
"Does anyone know anything about emissions?":https://www.mopedarmy.com/forums/discuss/1/382094/382094/
"Synthetic oil vs non: emissions":https://www.mopedarmy.com/forums/discuss/1/117577/117577/
If you really want to read those, I recommend flat view.