The GA 1 housing is no longer available...
The GA 2 from the G2 series is still available. However, the screw holes will not line up with the G1 unit you have.
You can buy the GA2 and drill some new holes in it to line up with the unit.
Or you can buy the GA2 and additionally buy the housing from an SR 300 G2 and fit it to your SR 300 G1.. then the GA2 will fit it...
Parts are available to order from here:
GA 2 rack adaptor
SR 300 G2 housing