Good work!.
That drive uses pwm by pulsing the drive enable pin, but since you can only get two outputs from the serial port, we dont have an output to turn on the enable.
One other choice to reduce the speed is to lower the voltage on the drive. Of course this may require a different type power supply.
You can also use a 555 circuit to pulse the drive enable.
This would require building a timer with the 555 IC chip.
You would need an astable mode circuit.
Here is a site with some info. You can replace R1 with a pot, which would allow you to adjust the pulses.
of course, this is entirely up to you.
One other way would be to add a larger gear on the monitor shaft. This would require re-positioning your motor to. Its kinda hit or miss to get the correct speed you want,
but if you are good at math (Im not), you could calculate how big the gears need to be.
here is that 555 site:
http://www.555-timer-circuits.com/operating-modes.html