The easiest repair is replacement from a salvage yard as used radius arms are inexpensive.
That said, I have straightened several as follows:
1. Remove the arms and reinstall in the inverted position such that the bow is downward.
2. Secure a length of chain to the rear of the arm at the socket and to the front axle at the arm attachment point. The chain should lay on the floor sufficiently to place a block of wood on top of the chain.
3. Place a suitably sized bottle jack on top of the block of wood beneath the downward facing bow in the radius arm. If the arms are badly bowed, a suitably sized length of angle iron cupped around the arm is helpful. Note: the arms are hollow.
4. Slowly extend the jack until the bow is slightly upward.
5. Strike the arm from above with a 2-4 lb. hammer a couple of times before releasing the jack.
6. Release the jack, remove the chain and reinvert the arms.