Rob, would this be a Scott Bonnar cylinder mower, with a manually-operated clutch in the drive shaft from the engine to the chaincase on the left side of the mower?
If so, you have to remove the engine to remove the right half of the clutch, then to thread something over the drive shaft on the left side of the clutch, you'll have to remove the left half of the clutch from the drive shaft as well. I haven't heard of anyone doing this except as part of an overhaul, which normally includes removing the drive shaft from the chain case. However I don't see why you couldn't remove the left half of the clutch once you've taken off the right half, leaving the drive shaft sitting in its bearing on the chaincase end.
Loosen the captive cotter pin in the right half of the clutch, or undo the grub screw if it has that instead of the captive cotter pin. Remove the four bolts that hold the engine down to the mounting plate(s). Disconnect the speed control cable. Slide the engine to the right as far as possible (until it hits the right side plate). If this separates the clutch completely without removing the right side of the clutch from the crankshaft extension, all the better - you won't have to remove the right side of the clutch from the crankshaft extension. If necessary though, slide the clutch-half to the left, off the crankshaft. Take the entire engine away and stash it temporarily. Next you have to slide the left half of the clutch off the drive shaft. It will be retained by something, probably a grub screw, and this will have to be loosened first.
Please take pictures as you go for our archive (I don't believe we've specifically covered the thrust bearing before), and if you get stuck or I'm wrong somewhere, show us a picture of the problem. A picture of the mangled thrust bearing would also be good.
