Previously this site helped me identify my engine using the engine number. Engine number says it is MC18 R2J. But then I found that the engine has a gear position sensor.

http://www.bikepics.com/pictures/2142437/

The MC18 has only a neutral switch. So taking a look at at the serial number again i noticed it has been modified. The old number has been ground out and a new one stamped into the engine case. The size of the numbers do not match and the aluminum has a divot you can feel with your finger.

Here is a photo of the crank.

http://www.bikepics.com/pictures/2142438/

Can this crank be verified as an MC21 via this photo?
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MC21R9N track
MC18R2J track
CBR954
CR250