I've done a lot more work on this. I wasn't happy with the HGLW chassis. Nothing wrong with the chassis itself, I like them, it just didn't look right with the Ruston. Up to now I've avoided making my own powered chassis. Because I'm a bodger, trying to get precision on mechanisms was something I struggled with. However, 3D printing allows me to design and make things with up to 0.01mm precision, so I gave it a go.
Since making this one, I've refined the design so it looks a lot neater. It uses a 12v 25GS370 (170RPM) geared motor and brass bevel gears. The wheels are Bachmann 24.5mm. It has Delrin chain drive for the unpowered axle.
The body now sits more realistically on the chassis.
The body is much wider than it should be, to fit the chassis. I'm now working on a more realistic version of the body - but the running plate will still have to be wider than prototype. My rationale is that, while Ruston extended their narrow chassis for most of their 3' gauge models, in the early days they did make bespoke chassis for 3' gauge models.
Rik