A few years back I replaced my aging rear master cylinder with a cheapo one. The seals shortly gave out. I looked over at my 2014 Grom and realized it had the exact same size and config as my mc28. I now have a Honda oem part that I have confidence the seals won't give out. Cost was just over $50.

My point is, don't buy a cheap one, but don't feel you got to get an mc21 part.