Hi Jap,
It is simply known as a "Clutch Body" and it is slid over the PTO (Power Take Off) shaft. It has a piece of square sectioned key steel to stop the body slipping on the shaft and grub screw to lock it on. Sadly the screws are not kept tight during in service life and the body becomes loose and being die cast ally it will wear quickly opposed to the steel PTO shaft.
You'll probably require a 3 leg puller to remove the old body off the shaft.
The following link is exactly what you'll require to purchase. You can find these parts in our online shop also. This is the complete clutch half with grub screws, thrust bearing and also a new piece of key steel.
BTW it's the 5/8 unit you'll require and not the 3/4 inch shaft sized body which is more commonly fitted to most of the much later Asian type engines with a 19mm output shaft (PTO)
http://www.ebay.com.au/itm/5-8-clutch-body-bearing-2-x-screws-1-x-shaft-key-scott-bonnar-and-rover-45/132535182941?hash=item1edbb6665d:g:vjsAAOSw~3pabTGi
Cheers,
BB.
