I went online a couple weeks back in search of a KN filter for the NX. Now I know that KN doesn't make a filter specifically for the NX-250, but they have a nifty tool allowing you to search by filter dimension. I found one filter that had similar dimensions to the stock unit.
Part #MG-0001 fits the Moto Guzzi V1000 LeMans/California models. It's 4"x6". It just came in the mail yesterday. I'll put up pics this afternoon on the install. Filtered area is somewhat smaller than the stock Honda unit but, I figure it's used on a 1000cc bike, so there shouldn't be a problem.
The only bad part is that this KN seems to be a hard to get item. KN doesn't show it in stock, so a call to one of the MG dealers would be required.
"I swear to god, it's like I live in a trailer of common sense, and stare out the window at a tornado of stupidity." -Roscowgo