Looking at the starter motor from the gear end, it should spin clockwise.
Most bike starters have two end caps and the cylinder with the magnets in it. All this is held together by long screws. I have seen some of the cylinders keyed the same 180º apart aparantly to allow the cylinder to be used on a bike that requires the starter to spin the other direction. I have also seen some brush plates that could be put in 180º out from the way they were installed or the brushes could be connected backwards. Either will cause the direction of rotation to reverse. Ask me how I know
I'm convinced that your starter is turning the wrong direction. The fact that it is engaging and turning, even though not starting the bike, should indicate the battery is in correctly. The electronics in the decomp unit wouldn't like reverse polarity and I doubt they would do anything other than maybe release a little magic smoke.
The starter spins clockwise, causing counter-clockwise on the gear next to it, causing clockwise on the gear next to the flywheel, causing counter-clockwise on the flywheel which if looking from that side of the bike would be the direction of engine rotation. The starting clutch only catches if the gear rotates this direction. The other way, it just spins.
Since you already have the cover off, try catching the flywheel gear and rotating it counter-clockwise. It should grab and try to turn the engine. Clockwise, it should spin pretty freely.
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