Welcome Philby.
Most of the stuff has been covered already - all I'd add is to cover some basics based on logic.
If the bike was running well when it was garaged, then one can assume that some of the 'other' things may be ok (valve clearances, points, magneto, etc, etc). If the motor turns, I'd whip the plugs and put a bit of engine oil in each hole to help lube the upper end by kicking it over gently. If there's a spark at the plugs, I would drain the engine oil by taking off the sump plate (checking the gauze for debris at the same time). After a petrol clean of the oil tank, I would put some cheapish engine oil in and, after a clean of the carb, see if it'll fire up. At this point, oil should start to return after a short while (maybe 30 secs or so??). The sound of the motor should give you more clues as to what you need to look at next (including exhaust smoke, etc).
As has been pointed out, the sludge trap is a concern after all this time, which is a full motor stripdown. I am not absolutely sure what effect a blocked sludge trap may do in the short term - possibly nothing, but if it's going to be ridden in anger, this would need looking at.
Would be very interesting to know more about the bike, like what model/year it is, what piston oversize it's on, what mileage is indicated, etc, etc.
The guys on this Forum love this sort of stuff, and can offer all the advice and guidance you'll ever need.
All the best with it mate!