Hi Alistair, Sorry to say I'm not able to be much help with undercarriage parts for that model, I have no parts at all left for on here.
Engine I can help with, Heck, I could probably find you a whole engine for it in the shed, but from the pulley on, I am beat.
However, once you have a full list of what you need, you could always pop a bulletin in the wanted section, and someone else here might well be able to help you out.
I'll hop on the phone to a few places and people i know tomorrow though, and see what parts are available for this model.
How is the engine going at this point?