LukasM, why not try Kawa KLE500 engine instead of Kawa KLE650 engine ?
I admit, it is only 500cc displacement, but it is proven, venerable design, bulletproof. Six gears, and it can run well above 170 kph in stock form. Easy to find, cheap to run & own, and with GPZ500 camshafts and carbs they develop a nice punch.
Same configuration as Versys/ER's, almost the same design, since Kawa 650 engine is based on it, but it should be smaller in overall dimensions and would maybe fit easier ?
They are carbed though, no factory EFI. You would have to do some aftermarket EFI conversion.
If you need measurements and photos of the engine, I can supply you with it, since I have one left from previous bike.