Wrong sprockets.
One has 1/2" bore; the other a 5/8" bore.
The pocket bike tranny's splines are .5118"(13mm). After you machine the splines off, you're down to .4331"(11mm).
You'll need to machine the shaft down to .375"(3/8").....
and use this sprocket:
http://www.ebay.com/itm/NIB-IDC-H35...066284?hash=item2a6c6330ac:g:8jUAAOSwaB5XiTpR