Bicycle Mechanics - no pedal threads in crank arm

I got this bike at the swap meet, and the threads on the left crank for the pedal are stripped out. causing the pedal to fall out half way through its first ride (the other half was riding home with one pedal, quite the experience)

basically wondering if there was a quick fix, or a way to just replace the one arm with one of my scrap bikes? or if I should replace the entire crankset.
thanks

1/2 or 9/16"? Which side. What kind of crank?

Your LBS should be able to install an insert or a Heli-coil.
Once in, just screw your pedal on.

Thanks guys,
I realized after my post how little information I gave you guys. It was the Left crank arm, and it had the square peg deal(sorry Im still not entirely familiar with what all these parts are called) I took it apart and saw how easy it was to pop off the arm with the right tools, and put on a new one. that was probably the easiest self learning repair/replacement Ive had yet, It just came right off and the new one just slid right on

Your LBS should be able to install an insert or a Heli-coil.
Once in, just screw your pedal on.

Depending on the quality of the cranks and how much your LBS charges for a helicoil it might not be worth it. You can get an entire crankset for ~ $30. Or your LBS might just have another crankarm in the same length, same Q-Factor floating around.

The helicoils for crank threads are expensive, like 10 bucks a piece and the labor as much as $40. Most shops don't have the kits either. Left crank arms, especially square taper ones can be had for much cheaper than the cost and trouble of trying to repair a hosed crank. Only expensive or novelty cranks are worth helicoiling IMO.