First I am giving the garage the benefit of the doubt as it can indacate a gasket problem but always thought it was mixing water with oil and not fuel unless internal leak to cylinders.
As for smoke from exhaust may be sheer co-incidental fault which the garage wouldn't know at the time. In my experience of same it's the oil control rings on the pistons worn. New oil control and compression rings on all pistons. Have experience all of this on car and motorcycle but fortunately of an age at the time I did the strip down and re-build on both myself, just paid for grinding, rings,pistons, small ends.big ends, gasket sets, and had the crank tested on mic it was ok.
Tje worst senario is a replacement re-furbed engine block using your existing cylinder head which will need re-face grinding to the engine block or you'll have another gasket blow out.
Unfortunately you'll have to put trust in your garage and if they have a good local reputation you won't get ripped off. Oh! one other I've even had a circlip break on a piston conrod and hitting cyclinder wearing a tapered slot in cylinder wall.
BB
January 2011