PLEASE HELP !!!!

Hi there one and all, i am and have been searching the internet long and hard for a piston for my 86' MC11 (56x50.6). I have come up with nothing! The closest i have come is the SA Version of the NS400R which used the same piston but guess what, none available.
Now not being the most mechanically sound person in the world i have been thinking about finding the clostest match piston i can and just seeing if it'll bloody fit. The closest i can get my hands on is Yamaha TZR125 (56x50.7) or Honda CR125 (56x50). Someone please give me some direction, cause searching the net over and over for the same thing is about as much fun as pulling teeth!
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Steve T
1986 NS250R MC11