Barry, it sounds as if it will be necessary to at least partly remove the engines from both mowers to measure the shaft sizes. Taking a good engine off a Diplomat to replace a bad engine on an SB45 does not, on the face of it, sound like the most economical way to replace the sick Kirby Lauson on the SB45, since it leaves a partly wrecked Diplomat behind. The engine SB fitted to the 17" Model 45 was the Briggs 80102, but I can't guarantee that it had the same output shaft size as the Kirby-Lauson engine, because SB seem to have had clutches with both 5/8" and 3/4" bore sizes. It will be useful for our archives if you measure both output shafts, and post the results here.
To measure the shaft sizes, you will need to loosen the clamp pin in the boss extending from the clutch on the engine side, if your model has a clamp pin. (Some had a grub screw holding the clutch boss to a key - if you have a grub screw, just loosen it a turn.) If you have a clamp pin, do not try to remove the pin, just loosen the nut and tap the nut end of the pin with a block of wood to loosen it. Then remove the four bolts holding the engine down to the mower deck. Hopefully you can then slide the engine half an inch away from the clutch to measure the shaft. Sometimes the engine shaft has rusted and jammed in the clutch bore. If that has happened you will need to put some Penetrene on the shaft and wait an hour, then try again.
