I seem to recall that back in my youth, when we used to fit 350LC engines to Gamma's, some folk replaced the rear wheel with early GSXR400 wheels, but can't seem to find any examples online..
The rear rim has a 2.15" rim width. So it's a really narrow one. I think you can squeeze a 2.50 onto it, but even 2.75 is a real stretch. Front is a 100/90-16", but it shouldn't be too hard to find a sticky for that.
Some nice photography there indeed! What about fitting a newer Suzuki rear end on that RG? One with a thicker wheel?
Or what about this one?
http://www.ebay.com.au/itm/SUZUKI-GSXR-400-1984-MOTORCYCLE-NOW-WRECKING-/110734891232?pt=AU_Motorcycle_Parts_Accessories&hash=item19c85058e0