I realize the 17211-358-000 filter is discontinued - it has been for quite some time - but I'd still try ordering it from a local Honda dealer. I got one of these about a year ago this way. It's surprising how much old stuff is floating around, waiting to be entered into dealer parts databases.
UNI Filters makes the NU-4012 for the '74 MT250. You may have to contact them to find a dealer who stocks or can order it:
info@unifilter.comIf you're desperate, this guy has a NOS filter cage:
http://www.tiaromotorcyclewreckers.com/ ... %20two.htmThe MT250 filter fits the MR175, so I wouldn't be surprised if the MR175 filter fit the MT250. dabbernz at eBay sells the UNI NU-4026 MR175 filter from time to time.
And there's always eBay, of course - an MT250 filter setup is bound to show up sooner or later.
Ray