This has been an 'enigma' over the last 5 years or so....
Ive seen it mentioned on many forums etc
There has been some success with R31 kits, with some modification....
ive seen pics of R31 GTS3 front bars straight on a R30, modified skirt, and GTS3 rear bar....
Jenesis (Japan)make them, and are pretty much the only place you can get them 'off the shelf'
I suggest finding a local place and asking if the do R31 kits, then take yours in for a measure & quote
if i ever get a kit, im going to get a R31 kit and cut&shut/plasic weld it, but its under the bonnet that matters to me right now