Progress Report:
Before I took my jackshaft apart, I looked at the end and it appeared to me that the shaft had an open keyway, so I ordered the sprocket with a built-in key. However, after I removed the driven pulley, I discovered that the keyway actually has a long piece of key stock that runs ALMOST the full length of the shaft. It didn't reach the end of the shaft that stuck out, so it appeared to me that the driven pulley and the spocket must have built-in keys. So, I'll have to exchange the sprocket with the built-in key for one with an open keyway.
This seems to be the correct part:
http://www.*****************/jackshft.htm
Part Number: 202154G
SPROCKET 9 TOOTH W/KEYWAY for #40,41,420 CHAIN
I had a hard time removing the old sprocket. The keystock that was present in the jackshaft was warped just in front of the sprocket. I hoped that I could force the sporcket past this bulge. I placed a rod behind it and started tapping, turning, tapping... I thought I was making progress as I got the sprocket to move a few inches. Then, I realized that the keystock was sliding out of the shaft along with the spocket. So, the keystock was sliding out of the shaft along with the sprocket. I'll also need to buy a piece of 3/16" key stock the full length of the shaft since the old one is warped.
I'm wondering how difficult it will be to slide that new keystock into the keyway in the shaft.