Re "how to power it", a good while ago I built two of the 7mm scale baby Rustons. One was a small motor & gearbox on each axle, while the other was a larger motor on one axle with Delrin chain to the second axle, as I recall both worked well. Highlevel gearboxes are your friend!
On the S4 society website I'm sure there's a section with loads of information on small coreless motors hung on axles with belt drive to gear them down. If that works in 4mm/S4 then it should be a doddle in 7mm scale!
Good luck John.