Sorry for the slow reply. Have eventually tracked down a good used camshaft (the end of which functions as the impeller shaft) and split the casing in order to replace it.
The mechanical seal includes a stainless steel cup that is tightly pressed into the casing and had loctite applied to it at the factory. The trick was to heat around it until it smoked (to break the loctite bond), and to then drive it out from the rear with a suitable socket.
There is an oil seal behind it, and both the oil seal and the mechanical seal cup came out together easily once heated. Doing it any other way can easily result in damage to bore for this seal.
The new seal cup was pressed in using a suitable 12-point socket with a rim that only contacted the outer lip of the seal cup; i.e. without touching the fragile spring-mounted mechanical seal face.
Seal faces were cleaned with isopropyl alcohol and lint-free cloth to a mirror finish, and then lubricated with fresh coolant (with a few drops of dish soap mixed in with it) prior to them being mated together during impeller installation. Also drilled and tapped the weep hole and installed a 90 degree elbow and short length of tube so that it now points downwards, as I feel that the forward-facing weep hole had the potential to allow dust to eventually work it's way into the mechanical seal cavity and accelerate wear.
I'm running the standard green coolant that's recommended, but am now wondering whether a low-silicate or silicate-free coolant (e.g. pink) would be kinder to the mechanical seal (less abrasive) and therefore prolong it's life. Curious if anybody has changed over from the green stuff to one of these modern water-pump friendly coolants...but in a small engine vs the typical automotive application of such coolants?