Others may disagree but IMO I would take the OB off and put it in the van if you have the space, just make sure you lay it down the right way up (assuming it's a 4 stroke)
As for putting it on the roof of your van, 68kg is a lot to be lifting so high? I put my SIB, 58kg, on the roof of my Navara and just about manage with 2 of us doing it.
__________________
|