As it happened I had a suitable chassis around, albeit with an electric motor in it. The electric motor and its gears were removed leaving a very usable chassis. I have opted to use meccano gears for the transmission, as they are easily available and should handle the load. Being brass they could wear a bit faster, but if they do replacement will be easy if required! It also meant I could research various meccano gearbox and clutch options. The one I based mine on was the "compact three speed and reverse" that I think was published in one of the meccano magazines. However I made it even more compact by altering it to a two speed without reverse, because I wanted a separate reversing gearbox so that it would be two speed in both directions. Finding a suitable reversing gearbox design to base mine upon proved somewhat difficult as most were based upon bevel gears which turned the drive through 90 degrees, and as the engines crankshaft was already parallel to the axle that would have been awkward.
So I developed my own design by having two gear trains on to the rear axle, one with two one inch gears, and one with three half inch gears. Therefore direction is changed by selecting one or the other gear train. The reversing gearbox is under the footplate, and the two speed gearbox will be above the footplate immediately behind the engine. The plan is that this will be concealed under the 'control desk' with gear change, clutch and throttle levers in a realistic place above.
well they say a picture tells a thousand words so...
Chassis with engine mounts and holes for gears
Underside, showing modified 1 inch and 1/2 inch meccano gears
Engine sat on chassis
And from other side showing removed flat belt pulley, because it was very wide!
Modified meccano pully mounted on rear of flywheel, and layshaft temporarily held in position
Mock up of two speed gearbox in meccano and ignition components that will be hidden under the footplate. Final gearbox casing to be made from brass once positions are finalised,