Unfortunately, once a wheel is cracked, we cannot repair it, and depending on the size and location of the crack would also not ordinarily recommend it to be repaired.
The only way to repair a cracked alloy wheel is to weld the crack, therefore covering the crack in new metal. If the crack is small and within the main body of the wheel, not on an edge, then welding may be acceptable if you can find someone who is recommended for welding alloy.
However, if the crack is on an edge, which is more typical, we would recommend you replace the wheel rather than have it welded.
This is because the temperature needed to weld the alloy makes the structure of the surrounding metal more brittle, meaning very often, as the edges of the wheel are under more stress, we see a new crack will appear directly next to the weld after a short period of time.