As seen before on my blog, Stew Ross has an S90 he is converting into an RSC. He sent me pictures regularly and we exchange ideas about our builds.
Here is one of the most recent pictures of his bike.
Keihin CR26 carb added and note the MT125R top crown plate with the racing brake and clutch levers.
Rencently I recieved some pictures from Mark Knowles, also from australia, and also building an S90 racebike. He has his engine all finished, bored and stroked to 124cc (54x54) with lightened cranck, clutch, rockers, valves, well almost everything actually.
I hope to see the progress as these bikes are being completed.