Help with CS 3/1

I have a 1939 CS 3/1, rad cooled and remote fuel tank. Its mostly complete, base plate, BKC 2kva alt. It is in need of piston rings and a small end pin, injector pump and injector. I am told that the small end pin will have to be made as they are not available any more.
I was thinking of up rating to a 5 or 6 /1 as parts are more available. What would be needed for this?
Would I be right in thinking that I would need a piston and cylinder along with a cylinder head. Would I need an injector pump and injector to suite the 5 or 6/1 or would any of the CS range of pumps and injectors be ok?
This engine I going to be used for light work running small machinery in my workshop.
Thanks
Simon