IJ,
The '74-'75 LH CR125M side panel won't fit MR175 frames without mods, and I'm not sure the rubber bands will line up with your '77 MR frame's hooks either.
What almost certainly
will fit your frame, though, is the '75-'76 CR250M cover, along with the MR250 and (duh) '77 MR175 covers; I think all of these are identical except for paint and decals. VMX Racing has them (about halfway down the page below, sold as a pair):
http://www.vmxracing.com/part-40.htmThis eBay listing shows both sides:
http://www.ebay.com/itm/75-HONDA-CR250- ... 76&vxp=mtrI'm planning on modifying my '75 MR frame to fit a '76 CR250M cover when I get the time. It'll take some grinding and welding, but should look good.
Ray