The MGA water pump changed during 15GB engine production from 2-bearing serviceable to cartridge bearing style. Early ones have a hex nut and woodruff key for the front hub while the impellor is pressed on and cross pinned. Later ones have both front hub and impellor press fit, no keys or pins.

The factory always supplied the shaft and impellor as an assembly, not separate parts. No need to remove the impellor from the shaft. Remove front hub, press the shaft out the back end, replace seal, and put the shaft back in. In between you can replace bearings in the housing if needed.

On 5/13/2022, Randall Everson in South Africa wrote:
"There are two bearing options available here, both regrettably from China. The older pumps used a 55mm long bearing cartridge whereas later pumps used a shorter 40mm bearing all shaft sizes are 16mm (5/8 inch). Interestingly the long unit costs about $9 and the short unit $3.5. I found these on Google search for “water pump spindle bearings.”
The mechanical seal is a standard issue but several suppliers here won't sell the fixed part without the ceramic mating ring. I did however find one selling Trisun seals yes also Chinese cost $5.5. The one part I can't find are impellers so I am reusing the old ones (may look at 3D printing in future)".

On 5/21/2022, Randall Everson wrote:
"The parts are quite common in fact the bearings are known by the dealers as water pump spindle bearings so I guess they are still in use in modern cars.
Same applies to the mechanical seals , once again I have come across 3 stockists here in South Africa all selling the same product but under different dealer part numbers. There seems to be an international category for these seals known by the Type 268, the common spec is by shaft size and metal body diameter VIZ 16mmx 36.5mm then in our case specify carbon seal and elastomer bellows.

The supplier will only sell what they call a two part seal ie it comes with a loose ceramic ring which would be the rotating part for other applications . We cant use this ceramic ring because the boss on our impellers is too small 29mm and the ceramic ring and housing is 32mm diameter, our MG seals have a carbon ring that seals on the end of the impeller boss. I have found the following in my area John Crane type 6A, another manufactured by Trisun under their own part number but the invoice description calls it 16mm type 268 carbon/cer/nit ( Cer amic Nit rile elastomer) another supplier here calls them Type B7 and their table shows F16 as 18mm hole (for a 16mm shaft) x 36.5mm. Other manufacturers are Kylin, Vulcan type75, Pac –Seal type 68 Etc. All Chinese producers but possibly available in the US.
A note on the sealing surface of the impeller after machining the damaged face it needs to be lapped in on a flat surface I use a piece of 20mm glass and fine grinding paste. See picture.
Now for the bearings so far I have purchased the cheaper of the two bearings part number 885118 made by Chinese company EFG, this is item D on my table the overall length is okay but the bearing is only 40mm long but shaft sizes are both 16mm. Because the overall length is okay it just means that the bearing has to be positioned further into the pump casting. Many of the original bearings had a 50mm long bearing I have located a supplier but it is on back order, this bearing is twice the cost of the 40mm unit and is known here as FPS061.
I have not been able to source bearings B and C, I have two pumps part number 12H4468 bearing C came from a Leyland manufactured pump with a 40mm long bearing the other is an aftermarket pump 12H4468 but has a 50mm long bearing so who knows which is correct.