Brian E - I see Mike gave you a link to the Charlie's FAQ's. If you want to see them all look to the top of this page for a link in the dark red letters. That takes you to the Forum FAQ's. At the top of the page with the Forum FAQ's is a link to Charlie's FAQ's, which is where Mike got the Spark Plug cross references.
For what it's worth I only ever used the Champion H10C. I think most others on here probably now recommend the Autolite 216 plug.
John Ware - here is a BIG handshake welcome to the IH Cub Cadet Forum. We don't get to many if any, original Owners on here. This is certainly the place you'll get some help and answers to your questions.
I'm gonna somewhat shy away from recommendations on your drive shaft since it's not my area of forte. I do think the problem at the rear of the shaft would most easily be addressed by just drilling a larger input and using the next size spring roll pin.
However, to completely resolve your problem I think you'll also have to re-address the front, which sounds like the problem started. The slot in the coupler is a commonly known wear item, but the key to long life is going back to use of an original solid pin. I'm surprised you got about 2 years out of a bolt. You may be able to get by with your current coupler but you'll need a good drive shaft (I suppose new) with accurate size hole. But lets also see what others provide for recommendations.
I know several knowledgeable members have done the rag joint conversions but not sure it has been done on a Narrow Frame 147. You could check the FAQ's link (dark red letters) at the top of this page, and then go to Charlie's FAQ's. I think there is discussion of a U=joint type conversion but that to me is really involved and you don't have the removable access cover on the frame making it much harder to get at the rear. I believe any conversion you attempt is going to require good accuracy and hard to do without that access. I came back under edit to provide you the link -
http://cubfaq.com/davek.html
I'll be checking back tomorrow for other responses. Good luck with your resolution here and make sure you come back often. We'd like to hear more of your original Owner experiences over the 42 years you've used your 147. And we'd certainly love to see some pictures of your 147.
Welcome again to the original and best IH Cub Cadet Forum on the internet.