if you lift the rear of the tank 10mm, you will need to support the middle of the tank (where it sits on the rubbers on the frame) with 5mm spacers. Otherwise the front mount takes a lot of weight and starts to crack eventually. I found out the hard way.
Yes it fits.
I have a Powerscreen engine in my 750L, and a fuel tank from an 1100 which has a slightly relocated tap.
When it had a 750 tank (which I damaged in a crash) I had to lift the rear tank mounting points a bit with some 10mm spacers.
Could be oil coming from the spark plug area, if the rocker cover gasket is not installed correctly/leaking. Easy to check, cheap to fix.
And yes Mk1 and Mk2 engines are a straight fit in the K6 frame.
Fuck yeah. Nice bit of Europe you've been riding around.
I was in the Ardennes last week, could have met you somewhere but I preferred to spend the little time I had with my family.