You could have the op amp make a square wavehttp://www.facstaff.bucknell.edu/mastascu/elessonshtml/UsefulCircuits/SqWaveGenerator.htm
and drive the 744410 with that as speed control on input 2.
Use a 2nd op amp as inverter to drive input 7. That will get you motor speed.
Use a 2nd pair of op amps to control which output is enabled, for direction.
I think the L293 datasheet shows the control relationships more clearly.
Pin 2 pulsing with 7 low is motor spin one direction. Pin 7 pulsing with 2 low is motor spin the other direction.
The wider the on-time of the pulses compared to low, the more the motor is on, with full speed when the output is just high.
Both 2 and 7 low or both high and the motor does not spin.http://www.ti.com/lit/ds/symlink/l293d.pdfhttp://www.ti.com/lit/ds/symlink/sn754410.pdf