Waldo, unless the previous owner altered the configuration from when I last saw it, I believe you have MC18 cylinders on that bike. Post a photo of the damage, and someone will identify it for you. They key to identification is a nice clear shot of the exhaust ports. MC16 ports are quite narrow/small, but the 18 looks like the map in this thread:

http://nsr250.net/forums/viewtopic.php?p=65898

Fitting MC16 or MC18 barrels is straightforward, but fitting MC21 or MC28 will also require their respective head(s) and altering the plumbing. Far from a difficult job, but it could be a bit of a pain.
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Andy.
NSR-WORLD.COM

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