G'Day Neil, no mods are required mate, my GS cooler will fit all the GS models from the 750 to the 1100. I was making them long before I started on onda coolers.
I think (but aren't sure) that there are free online manuals for the GS' on The GS Resources site too.
I've got parts microfische cards for every Suzuki made from 1972 to 1988, and when I had access to a Defence Department reader/printer I could have made you a copy, but that was a few years ago, sadly. I've heard you can get something that will scan "fische" cards like a photo scanner, but don't know much about that. If you do, let me know?
Have fun on that GS Neil, and you're quite right, if you find that the 750 engine is too tame for you, good news is the 1100/1150 engine will fit in your frame, then you'll have a 7/11! Cheers, Terry.