Hi Jhon,
I put a schematic of the adapter ring in Appx. II of the RAINFOR-GEM manual (footnote, p.72). If you extract that image and blow it up, you get the attached. With that, you should be able to get an adapter ring made up in any workshop (best to take an SRC-1 with you to the workshop so that they can make it fit exactly).
Saludos,
Toby