Well you need to be a little more specific about that, but as far as i can make of it, its going to be pretty difficult doing what you asked.
Instead you could replace both the leg motors with just one simple DC motor. But you'll have to feed the DC motor with inputs from both the leg motors along with using simple diodes.
A more detailed way of putting it is:
- Remove both leg motors and replace with just one DC motor.
- Now take the input of 'leg motor-1' and connect it to the input of the DC motor via a diode.
- Similarly take the input of 'leg motor-2' and connect it to the input of the DC motor via another diode.
This should feed only positive/forward inputs to the DC motor while the negative/backward inputs are cancelled by the diode.
---------- Post added at 09:21 ---------- Previous post was at 09:11 ----------
OR [ A much less complicated way ]
Assume we ignore/remove 'leg motor-2' altogether and we will use only the input of the 'leg motor-1'.
- Remove the 'leg motor-1' and connect its input to the DC motor via a 'Rectifier Circuit'.
[Should do the trick]
If the voltage to the DC motor is not sufficient enough, you could use a simple 741 amplifier circuit with an external battery as power source.