Well from the pictures i've seen of that machine there's nothing special there.?

Do you mean you can't replace Like for Like as in manufacturer of the stepper or you can't actually remove it from machine.?
You don't have to replace like for like and any stepper make provided it's sized correctly and matched to your drive will work fine.

That said I'm 99% sure it won't be the stepper causeing your lack of accuracy but more likely a worn ballscrew/nut or bearing with play in it, could be as simple as worn motor coupling. Carefull examination of Z axis will show any signs of play or movement from ballnut/screw.

I certainly wouldn't call it scrap or even close to being scrap and if it is then I certainly wouldn't be buying another from same company after only 3yrs and lite duty usage.!

Unfortunatly your a little to far away from me otherwise I'd gladly look it over for you.