What is wrong with an XT660?

Personally, an air-cooled XT600 or a Suzuki 650 air/oil-cooled machine, either Suzuki, CCM or Sachs could be on the cards, with the Suzuki motor. This motor can be taken out to 790cc, with a ProCycle kit:
ProCycle_790cc_Suzuki_singleNot sure about reliability or total cost, also, apart from the Sachs the other machines may not take kindly to 2 "biggish" persona aboard.
Another option might be to go the Suzuki 'Savage' 650 single route, with a enlarged bore kit fitted.
Something to mull over and kick around as you wait for the traffic to move on the M?? when waiting for 2cm of snow to be cleared or the lorries are backed up from the Chunnel to the M25 parking circle and beyond!!

A facetious poke!

Nochd math, Bill