You will not be able to reproduce the exact same effect than a precomputed renderer like Arnold. BUT, as usual in realtime, you can fake it!
An old but/and efficient way is simply to model and texture the light effect, like in this example: Richard's Art Gallery - Audio Tour - Download Free 3D model by rlockett (@rlockett) - Sketchfab
Your FPS will love it.
Note that the effect you want may be named “volumetric fog” rather than “god rays”. Example through the Unreal doc’: Volumetric Fog | Unreal Engine Documentation (Note also that this was add to Unreal only on the 4.16 version, so it’s relatively new, and I’m not sure this effect will be availabled easily through webGL?).