For a first try, I take the configuration of the manual viewer as layout. I need two stepper motors. The first one is connected to the wheel with tooth that grab into the holes in the film. It has 12 tooth, so 1/12 turn of this wheel advances one frame. The second stepper motor (although it could have been a normal motor) turns the spool. It should be able to slip, so I made a few different clutches.

I used the simple 28BYJ-48 geared stepper motors with ULN2003 drivers.  

I made a frame and holders for the stepper motors in OpenScad and sliced it with Cura.

The best clutch was an outer ring with spring-like pins, and an inside wheel with an curved surface.

Problems with this first prototype: 

• The film was too loose around the wheel, so it slipped. 
• The microscope holder was off. 
• And it was too dense to tweak. 

So the second prototype has to be wider, several options for the guarding wheels to get enough tension, and a proper place for the microscope.