The OEM Keihin carb on the MR175 (all years) has a 35mm spigot, 44mm mouth, and 98mm overall length. The Honda MT250, FL250, and '74-'75 CR125M carbs are nearly identical.
If you want to install a new replacement carb, the closest physical fit should be the Mikuni VM28 (35mm / 44mm / 95mm are the dimensions I found - if you do decide to get one, double-check the dimensions with the seller in case mine are incorrect).
http://www.ebay.com/itm/Mikuni-Geniune- ... 78&vxp=mtrThe Keihin PWK28 or PE28 would probably be my personal choice, but their dimensions (35/50/83) make installation more difficult. There are very cheap copies of these carbs available on eBay; I have no idea how well they'll work, or for how long.
Ray