Can i use my old rc car curcuit board on the rc helicoper i am trying to build ?

Perhaps.
[Haven't been involved with this for a Number of years, so bear with me]

You need the correct number of channels first of all.
Each channel operates something on the radio guided craft.
1.You need 1 channel to vary the throttle speed of the engine. (Makes the Main Prop, and the Tail Rotor spin faster or slower)
2.You need 1 channel to tilt the Prop.
[To go forward]
3.You need 1 channel to move the tail rotor vane. [To turn]

If the radio controlled RC car had the options, 1.Of how fast it would go, (1 channel),
2.The wheels turned, (1 Channel),
3.Plus had Lift, (1 Channel), you're good to go so far.
[Lift = RC cars that had option to raise the front end up, like a real car with hydraulics]

Next comes the question, of how far the radio controlled car's transmitter will transmit.
You not only have to take into consideration of how Far away from you, but also how far UP.

Fly out of range, and your 'bird' will drop to the ground like a Rock! (No wings to glide down)