Has anyone tried downsizing tender-drives yet?

A smaller motor/battery isn't enough. Those C battery tenders (E/H/G/J) are clearly too short for this. In addition to the motor, they also need a gearbox (for torque, going up slopes, etc.) and a unique type of on/off switch that dis/engages with it (for the ability to freewheel). While there might be room for an N20 gear motor +battery in a shortened tender, the driven axle would still be permanently engaged. In Emily's case, the tender is too tall but not too long.