Spray a good load of wd40 all over it, and just kick it over every now and then without starting it just to move the piston to another position.
Drain any fuel out of it also and make sure that it isn't just water in the cooling system.
If you are close by I am happy to take it for a spin every now and then.
The important thing is that if you do start it make sure it is run until hot, not just warm as this will cause build up of moisture in the engine.
Take the spark plugs out and kick it over several times to circulate the oil (make sure there is oil in it!!) Do this regularly - at least once a month and remember to put the plugs back again. A standing engine will allow oil to drain down into the crank case and over a long time the pistons, rings and valve gear can rust due to lack of oil. Also check gearbox oil - even a slight leak can empty the box and ruin the clutch and shaft bearings. Block up the carburretor air intakes (but take it out when you spin the enging over) - this will stop condensation getting in.
Don't worry about cooling fluid - it's air-cooled.
Just had a quick look round some dealers: price is in the £6,000 to £8,000 range.
Lovely machine!! I had one of the first 'unit construction' (engine and gearbox made as one unit) Bonnevilles - 'B' reg. Paid £265 for it second hand when it was just over a year old. If only I'd kept it.......