The Rotax engines are made for the leisure industry (snow mobils i think), they are therefore not made to racing engine tolerances. This means some are alot better than others! the main difference is the casting of the barrel/ports. If your buying an older engine the '07 is the one to have although there is no guarantee that makes it quick. If you have the money get a new '09 engine as the barrels on these are now all CNC machined which ensures they are all equal and quick (well as quick as an 07 engine). An engine with an 09 clutch would also be better as these are designed to last longer than the old style cluth (no performance advantage though). A good carb and exhaust is also important though. Make sure any exhaust you buy isnt covered in dents, and/or cracks! Hope that helps!
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