This is how the original lighting/rendering looks like.

- adjusted curves to get some more contrast
- added something like this
- and a very transparent yellow layer in front of all
- the volumelight is the fR-Effect

The fR-Settings are simple:
- fR-Image with Reuse-Solution
- 5? Diffuse Bounces
- 200-300 RH-Rays
- 70-80 Balance
- much lower Curve-Balance
- min/max Density adjusted with "Show Samples" on until the green spots where 1-4 pixels in size ... and min/max at a ratio 1:10

- the sun is a parallel spot. I played around with the multiplier until the lit floor-areas where almost white
- the background is white although I turned off consider background in the fR-Globals. I used only skylight instead. (but this makes no different) The white background was needed for the blooming.