You may get away with it, depends whether it has damaged the crank or threads inside the crank where the bolt goes when it came off. Has the bolt snapped or just worked lose? Presume it must have had the pulley off at some point and not been tightened up right.

I once had one like this on a Shogun, he wasn't as lucky as you though as the pulled launched itself through the radiators!

Well just been to the garage to see how it's getting on. They had it all sorted a new tapered washer on the retaining nut. It was out for a test drive and on the way back it spat the pulley off again.
Can anyone confirm the torque value for it. I had 270 nm 200 lbs. ft
Any reason why it would want to back off the retaining nut and then the pulley?

Have they used thread lock? Inspected the threads in the crank for damage / made sure the end of the crank isnt damaged / gone oval?

Not sure if its the way the photo is taken, but to me it looks like some of the threads have pulled out of the inside, and the end of the crank doesn't look so good.

By the way, if it has pulled the threads out or damaged the I have known engineering companies come out and drill / helicoil in situ with excellent results.