I think your Greenfield is similar to this one:
https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=20076&page=1

Deadly Dave and Murray set out the process in quite a lot of detail in that thread. They can probably help - I may even be able to myself, if the questions are easy enough. I suggest you post some pictures as you go along, both to clarify what you are doing, and to help other people later.

And the sprocket retaining screw is essential. If you don't have it the chain will be damaged when the sprocket walks sideways in the outboard direction. Unless it is very tight on the shaft, in a surprisingly short time it will damage the chain enough to allow the sprocket to fall off.