Firstly, the 18rg one is much smaller than the 18rc one.
The mountings on the 18rc one are on the other side of the compressor than the 18rg one, this would make like very difficult.
The 18rg has a small, cylindrical pump, I have seen similar ones in wreckers with similar mounting points to the 18rg ones.
I think it would be easier to find a cylindrical, small compressor from another engine than to adapt the 18rc one.
All your piping will need to be modified too, as the 18rc piping does not reach the 18rg compressor position, so you may as well just do the whole thing.