http://www2.snapfish.com/snapfish/slideshow/AlbumID=5671603007/PictureID=284993651007/a=59315083_59315083/share_version=version_B/

Engine # is MC16E - 1032338. I searched "engine" and "number" and found a reply from RobH indicating this number range to be within the R2J (MC18) range. I also found multiple references to the parts schematics having frame and engine numbers but could not find them there.

Top barrel is hexagon and fits the engine case perfectly indicating MC18.

Front barrel is round (MC21) but does not marry the base quite right. There is a small triangular section of engine base machined flat that looks like it should be covered by the barrel, but is not. Again indicating MC18. Is it possible to install a MC21 barrel on a MC18 case?

Yet magneto seems to be from MC21 and not MC18. Again, is it possible to install a MC21 magneto on a MC18?

This is all very confusing for me. Can anyone help?
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MC21R9N track
MC18R2J track
CBR954
CR250