Hi, and welcome to the USA !
Here's the Michigan DMV page - http://www.michigan.gov/sos/0,1607,7-127-1585_1587_1670_47674-175972--,00.html
I assume that if you already have the title/registration from Germany they should be able to issue you the appropriate state documents, but it's government so who knows?
Driving your Quickly to Niagara Falls is total madness. Take pictures and video when you do it. I'd like to drive my Honda Cub from Key West to Alaska some day, which is far more foolish.
As far as crossing the border with the bike, I don't think that would be a problem with A U.S. license, title, registration, etc.. provided your visa allows you to leave and re-enter the U.S. That might be a bigger concern.
Since they'll probably want to look at the bike at the DMV, and since it's just a good idea, get some mirrors and a horn on the bike, regardless of whether or not your model was made with them. On the other hand, the people at the DMV won't have the remotest idea of what they're looking at. That can make things easier or harder.
I did a quick search for NSU parts, but didn't see any sources in the US. This one is in the UK - http://www.nsuquicklyspares.co.uk/
There are a few people here from Michigan, hopefully they'll offer somewhat better advice :)