Make sure you get the 20mm shaft.
https://www.amazon.com/dp/B098X7HNC3
13-17T fits without any modification, just plug and play.
For an 18T, you'll need to grind a little off of the top left of the sprocket cover (near the starter motor).
For a 19T, you'll need to grind off the left 1/4in of the sprocket cover, and a bit on the engine case.
A 20T doesn't fit without major case grinding, and welding.