The closest Mikuni I found was the VM27.7, which is 5mm shorter but otherwise the same as the 28mm Keihin.
For what it's worth - it took a while, but I finally managed to find some eBay 28mm Keihins that hadn't been drilled out, packed solid with dirt, or wailed on by impact drivers. When the carb is set up right, it really works quite well. The MR175, '74-'76 MT250, and '77-'84 FL250 all use the same basic carb, with slightly different jetting and needles.
Here's an MR175 carb auction, from a seller I've had very good dealings with:
http://cgi.ebay.com/ebaymotors/HONDA-MR ... ccessoriesRay