pure speculation report, page-13

you raise a good point the advantage with using a reciprocating plate feeder on a feed hopper is that simply by changing the stroke length the feed rate can be changed.

Alternatively instead of using a motor direct coupled to a gearbox an off the shelf electric motor is used with a pulley which runs a belt to the driven pulley on the conveyer. By changing the pulley sizes the conveyer speed is easily changed similar to a home garden rotary hoe set up.
Would take around 2 hours to fabricate the motor mounts (sorry for being flippant) but it was my line of work once.