Microcontroller ?
How about an RS232 line driver (for voltage shifting).
Serial data (now 0 -- 5v) into microcontroller.
Microcontroller decodes (simple shift register job) to parallel.
Microcontroller uses 7 bits to output an analogue (128 steps) value for speed to a power transistor as an emitter follower (allows higher current).
Microcontroller outputs 8th bit to a relay for reversing the motor.
If the microcontroller can output analogue then magic, if not then build a simple D-A with 7 binary-weighted resistors and an op-amp buffer.
The lot could be as small as 3 off 8 pin ICs and a transistor. Hey, if this is single direction RS232 then skip the line driver and use a resistor and zener ?
I was thinking of suggesting my old favourite the 6402 UART and a host of other bits but the above could be more elegant ?
##################
What's this thing about serial being harder to program than parallel ?
I reckon the opposite is true, my pocket PC serial port is easy in BASIC, I just send it a number or character. The parallel port (printer) is unreachable!
Similar thing on my Sinclair ZX81 (now in storage) and Amiga A500+; all proper computers, not like this Windoze/Intel conspiracy !!
##################