Our reel mower guru, Deejay, is the right person to answer your questions. It is very unlikely you will need to abandon the Thoroughbred over a mere engine/clutch shaft sprocket: even if you had to have one made, it should not be a major expense, but more likely than not it will be possible to find some kind of standard sprocket that can be reworked to suit your mower. The first step is to count the teeth on that sprocket, then see what size the shaft is where it goes into the bore of the sprocket, and how the two are attached together. Most likely you will find that the retaining nut has a right hand thread, and there is a key between the sprocket and the shaft. If that nut has a left hand thread, there probably isn't a key, but this is less likely: the left hand thread is typically used if the inside of the sprocket itself is threaded, and there is no retaining nut. Sprockets normally have positive drive, not friction drive.
You can put the Kirby engine on either mower, and find a Briggs of suitable age for the other. Usually a 14" mower would use a Briggs 60102 (2 hp), and a 17" would use a Briggs 80102 (3 hp). Both engines are quite commonly available second-hand.
I think you have two very restorable mowers there, and the right answer will be to restore both of them. The SB looks to be in practically mint condition, and I think will be the more valuable of the two after restoration: it is a very popular mower.
