I tend to sit the gearmotor in a folded brass 'U', and hold it there with a couple of spots of 5 minute epoxy, which are easily cut out if I need to remove the motor. To be honest, and I know this will make people cringe, but I've always found meshing these gearmotors by eye perfectly satisfactory (with bevel gears).
Get some weight into the ends of the tanks, or you will have pick up problems on the leading and trailing axles (all the boiler weight coming over the middle axles.)
Technobots sell black bevel gears at 45p each (about 12mm dia, also 2mm bore)