Do not expect to find anything to mod like the DT125,i dont think it is worth the effort.
I had read here at the forum that MC18 has better chambers from the MC21.
As i know the restriction is in the silencer.
Only MC28 has a washer restriction on the chambers.
Check here the differences on a MC18 (R5K-R6K) between the pipes. http://www.nsr-world.com/tuning/250-tuning/ignition/
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Owner of : Honda NSR250 MC28 95sp colours / Honda NSR250 MC18 R6K SP / Honda RS250 NF5 89 in boxes
/ Honda C50 12V cub / Honda C50 12V super cub Japanese version /Yamaha XT600 89 benetton