YSS do a shock to fit, bolts straight in and seems well made. Sorry I don't have the code, I can find it on the work pc maybe but you should find it listed on their website or a bay of e search for GS1150/GSX1100EF. I seem to remember bolting a Hagon into my previous EF without any problems too.
Either will improve the handling over the 35-year old original bouncy spring, as would the 2x4 mentioned above