Have a think about this:
It would be hugely impractical to use an electric starter motor to start an aircrafts main engines, the starter motor itself and the battery bank needed to power it would have to be big (read - heavy). So an APU is used. An APU (auxilliary power unit) is a pint sized gas turbine started by a small (car size) starter motor from a small battery. The APU has driveshafts that connect to the main engines to spool them up for starting. They also drive the hydraulic pumps and generators whilst on the ground. If there’s a problem with the cross/coupling shafts, the jet efflux from the APU’s exhaust can be directed onto the main engines turbines, spooling the main engines up fast enough to start them. Typically mid-sized APU’s produce around 250kw of power, equating to just over 300 shaft hp.
A mid-sized APU fully dressed with starter motor + generator/alternator, weighs less than 50kgs (about the same as the coolant alone in a lot of trucks). Smaller ones look almost toy/model like!
They use their own, self generated compressed air supply for cooling.
They can run on anything that burns: cooking oil, waste engine oils, animal fats, meths, turps, petrol, paints, gas, anything you can pump into them. They can even run on fine solids (coal dust). If it burns, and you can pump it into the engine, it will run.
So my thinking is, directly couple an APU to a good sized hydraulic pump, such as those found in an excavator.
Use the hyd pump to power a hyd motor, directly coupled to the diff. The motor would be infinately adjustable from 0 - 2000 rpm.
You would remove from a standard tractor: Coolant, rad, intercooler, water pump, header tank, starter motor, 1 battery, alternator, ps pump and resevoir, engine, manifolds, turbo, exhaust, clutch pedal, master cylinder, slave cylinder and clutch assy, bellhousing, gearbox, gear stick and linkage, propshaft, pto and retarder (if fitted). Whole weight, I would estimate 1.5 tons.
You would add: APU, hyd pump, hyd resevoir and oil, hyd oil cooler (in place of rad), hyd motor, associated valves and pipework.
Whole weight of this kit, I would estimate 300kgs.
The APU would have minimal engine control (simply running or not) and would run at a constant speed with no ‘in cab’ controls as such. The throttle pedal would control the oil flow through the hyd motor so it would drive just like an auto. The hyd motor would ■■■■■■ just as well as it would drive, so no modification needed to standard vehicle brakes. In fact, it would probably need some sort of ‘soft start’ device or torque converter, or you’d spin the wheels every time the throttle was nailed.
Main practical problem would be, where to route the exhaust. Point it downwards and when stationary, you’ll literally set the tarmac on fire. Out to the sides you’ll toast any parked cars, pedestrians, motor biker ists. Upwards, it would have to be a long way up to prevent the trailer taking fire. You wouldn’t want to pull a decker. Mind, most folk wouldn’t want to pull a decker 
Good OP.
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