I have seen a few times where it has been stated that, in lieu of a F600 class, a bike motored F500 conversion can be raced in FS. Now that the new GCR is out, I thought I would read the FS rule set. Turns out that FS has a few funny requirements. First of all, their suspension system requires "springs" (I didn't check to see if the GCR defines rubber chunks as a spring). Second, the minimum weight is 750 lbs without driver. This would probably require ballast in most F600 builds. Finally, they appear to require mirrors substantially larger than most formula car mirrors. There may also be a few of the chassis construction rules that diverge from F500.
Looks like nothing is ever easy.
Marty