Interesting undertaking!
Finding commercially available parts shouldn't be impossible.
But as a start, it would be helpful to have a drawing of the original thing with wheels diameter, wheel base and the dimensions of the "blind shaft" in the middle, etc.
We could then look for these parts, for example in the Märklin 1 gauge or LGB spares inventory
Connecting rods could also be made in 3D printing, but I doubt you can achieve the required accuracy for a real smooth driving operation. (Typical 3D printing of 0.1 mm accuracy won't do it here!)
But these parts could easily be machine out of brass profiles.
Is the real thing a standard gauge (1435 mm) locomotive?
If yes, since G gauge is actually 1/22.5 scale there could be a disconnect here, as the 45 mm LGB track corresponds to 1000 mm narrow gauge at that scale
Are you rather talking about 1 gauge, i.e: 1/32 ?
My two cents..
looking forward to see the drawings
Cheers