Thanks all for the replies!
No idea, I'm afraid. Once I source another engine I'll be able to compare cams and head height to get some clarity, i guess.
"ECU is also mapped differently. Versys ignition timing ranges from 10° BTDC @ 1300 rpm, to 33° BTDC @ 5000+ rpm.
650R: From 10° BTDC @ 1300 rpm, to 35° BTDC @ 4800+ rpm."
I don't have enough experience to tell whether this is a small or a big difference in timing (i.e. if the versys ECU would work), but at least it's starting to sound like ER6 engine + ECU would be a workable solution.
More googling required (ECU fit, mostly)