Most parallel ports on most computers (Amiga, C64, PC, etc...) have (at least) 8 true digital pins that one can interface with for either input or output. This can be extended using shift registers or multiplexers; I've written up an example of this here.
With a lot more pins at one's disposal, more items can be controlled; as long as what you want to control is in the digital realm. To control a standard DC motor, for example, you'll need to be in the analog world and we'll describe how to get there below.
Digital to Analog Converters
The basic principal is to use a number of digital inputs and map these to relevant resistance values on an output. If you had 4 pins available, then you could determine the maximum resistance you needed and divide by the number of pins. You'd then make sure that, as each pin was brought HIGH, that the resistance summed towards the final value that you required.
You can either use a bunch of resistors to do this, or an integrated DAC circuit.
This circuit consists of a bunch of resisters in parallel/serial, using properties of such combinations to provide a stepped resistance output. This is known as an R/2R Ladder and is a popular method for converting digital signals to analog.
Combining it into our Parallel Port Interface
The 8 outputs from the 595 need to be de-coupled from the LEDs and provided as the inputs to the resistor ladder.
As that we can send any value to the 595, you don't really need to be careful as to which end is the MSB, but do remind yourself of it as it'll become important when writing the software.
You should now be able to check the voltage on pins 1 or 2 to see the variance between 0 and 5v when you're switching the bits on and off.
To my surprise, the output voltage was nearly exactly double the byte value being output by both the Commodore 64 and the Windows Parallel Port.
Once the test breadboard produced the result I wanted, I confirmed it all by soldering it together on the PCB. Not the ugliest mess I've created, but not very far off! And... it works.
Now that we have an analog interface, we can control a PWM throttle. To do this, we'll go back to the Arduino world and steal a motor controller. I've previously worked with H-Bridges before and will use a module to make this easy. Controlling direction will be easy also!