[Greg Perkins]

i had this problem too a while back, heres what i did... i got some PB "nut buster" and douced the heck out of the keyway/shaft and i let i sit for about 3 hours. then i came back and having the shaft vertically pointing up (mount the motor temporarily so the output shaft is vertical)
, i got a peice or square (or round) stock thats bigger than the shaft but smaller rthan the OD and got a peice of wood and gave it a couple of good whacks w. my 30lb sledge
the sprocket came off the key, and then i was able to remove the key and slide the sprocket of...be wary though you may shatter the sprocket or even bend it.

you could use an oxy-acetelene torch and heat the hell out of it and then beat it off try and get a few pics too...