I swear the cases look very close to a GS500.  I think the cylinders are a tad taller.  That RD 400 swap has me wondering if this would make a worthwhile swap project.  650 motor into the 500 frame?  Any thoughts?

Looks like someone has started to build one;
http://gstwins.com/gsboard/index.php?topic=24516.5

No finished posted, but he says it fit in and he was working on fabbing electrical/exhaust. A bolt in swap would be cool to see done and documented!!

The GR motor is about 1.5" taller, and shorter front to rear (to make room for the "new at the time" monoshock suspension).

I've never swapped a GR / GS motor, but I do know a bit about the GR650.

Why? It puts out the same power and doesn't rev as high. It's a pretty lazy engine. The GS engine is probably a step up.

For racing, the GS engine is the way to go. Parts availability,  20+ years proven design, etc.. If I were to pick between the 500 and the 650 for my "daily beater / cross country tourer", the GS is a no brainer. But the GR motor is a different animal: more torque, more power, and the dual stage flywheel make for much more fun below 100mph.

also the head is different. it wlil ALMOST go directly in, ( i tried, and buddha did as well.) the exhaust exits at 45 degrees it would hit the frame. some frame fab needed or a head swap. buddha can fill you in more on this. also only sold in us for 1 year. p[arts are SCARCE. took me months to locate forks.

No pictures?