Well, some good and bad news. I found a drive coupler inside the pump that extends the motor shaft enough to mount a sprocket on. The bad news is that the bolts holding the end of the motor onto the pump are either 4.5mm, or imperial. My friend has some imperial allen keys, so we'll have a look. Even worse is the fact that it looks like the bearing is held by the pump
However, I'll take it apart and see what I can do. I could always extend the motor shaft through the pump housing and add a bearing to support it, or lathe up a new end cap. I have lots of time, as I finish my GCSE exams in a few weeks, and have a 3 month holiday.