Rotax genuine sprockets have "ROTAX" stamped on them!
Use a caliper and measure its width, that might give away the reason for your tight drum. Does it spin lose without the nut fully tight?
Youcan't always just go to the 13t front! Here's why: 12/82 = 6.833 (ratio) 13/89 = 6.846 With an 89 sprocket at the back, depending on the ride height you might be bottoming-out on left handers, which will affect handling tremendously.
You can "manufacture" a tool in the vice to remove the sprocket. Fasten a couple of big bolts in the vice which would fit both through the middle of the sprocket and one of the holes in the drum, then turn. Make sure you don't damage the inside of the sprocket or you'll be asking for trouble later. But you get the idea!
Dan
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