Most electric vehicles have minimal gearing certainly a 100-150v 55KW electric motor as used in the Piaggio would be enough to drive your Hi-jet. The reduction of the back axle being sufficient for the motor to perform adequately on direct drive.

The electric motors used are of the shunt wound type which means they are to a degree self regulating when under load. Take one off load and it will spin up to self destruct speed very quickly.

Which is why electric vehicles have very limited towing speeds and long downhill runs need to use electric and friction braking if the motor is to survive. On over-run the gearing works the other way and multiplies the motor speed rather than dividing it.

So expect to spend a good deal on a sophisticated motor controller. You will need to weigh or find out the weight of the engine gearbox and fuel tank. This will give you the weight of Motor and drive battery you can use without affecting payload.

Modern traction batteries are quite light and designed for their task; do not use car batteries as they are not really designed for the heavy charge discharge cycles that traction cells are set up for and will probably fail quite early on into your project once it is out and about.

Current flow is incredibly high and several hundred amps can be drawn to start a fully laden vehicle from a halt. A 1/12 scale electric racer weighing about 2.5Kg has a speed controller capable of handling 25Ampere which is the current that passes in getting the car rolling. Work that up to a fully laden micro-van and you can see that this figure could be much closer to 500-700Ampere put the vehicle on a slope and that demand goes up again.

Alternatively fit as smaller engine say a 250cc motorbike unit set to run at 3-5000 rpm and use that to drive a small alternator. Use a shunt wound AC traction motor to drive the vehicle and develop a hybrid drive. The overall efficiency will be low but better than the original van in energy terms as the fuel consumption will be much lower.

You still need sophistcated control but the load of batteries is nil other than the unit to start the bike engine so the standard van battery would be sufficient using on-board generation means that you can tap the traction power to charge the starter battery so the payload would benefit by possibly 200Kg.

If you want to use gearing to ease the power demand in either case I would opt for a step-less design as used in some cars and live with the lower operating speed and duration...

Good luck with the project

1st 3.769 (43/14 x 27/22)
2nd 2.045 (35/21 x 27/22)
3rd 1.376 (37/33 x 27/22)
4th 1.000 (-)
5th 0.838 (28/41 x 27/22)
Rev. 4.128 (37/26 x 26/11 x 27/22)
Diff 4.444 (40/9)