starting up a long unused DT 54 tractor/dozer... they have what sounds like a small motorcycle or snow mobile engine

this is called a pony motor (although John Deere calls it a "Cranking Engine") and it cranks the main engine with a pinion gear just like an electric starter.

 This one is a two lever unit - one lever (on the left) to engage the drive pinion into the ring gear on the main engine. This would be done while the pony motor was warming up.

 Once the pony was warm the second lever would engage a clutch between the pony motor and the drive pinion to turn the main engine. (There are also three lever models where the third lever would allow you to select either high or low gear on the pony motor.)

 Once the main engine started it would automatically kick out the drive pinion from the pony. In cold weather the pony motor may have to turn the main engine for 20 or 30 minutes before starting. The pony motor shared its cooling system with the main engine and the exhaust pipe from the pony was routed through the intake manifold for the main engine in order to warm it up.

 This was a great system for a few reasons. First the main engine never started without first building up oil pressure. Second, since it took some time to start, the operator had an opportunity to check the machine over and grease fittings while waiting for the machine to warm up. Thirdly, time was on your side. The longer the pony motor cranked, the more likely the main engine was to start (unlike electric starters).

then they used the running dozer to bump start the 2nd dozer that doesn't seem to have a starter motor