How to Change a clutch plate on Webb Lawn mower?

I have a Webb 14" Lawnmower and the clutch plate has disintegrated. I have bought a new one from ebay but how do I fit it ?
I started to take the clutch shaft out but think this is wrong way and should unbolt the engine and slide it along allowing me to slip in the new clutch plate.
Can anyone give me some advice PLEASE
Thanks,