You dont need to change the crank, Just get a new pulley and replace it,
IF the nut of the Pulley is broken half, and half of it is stuck in the crank. Only then you need to replace the crank.
I got it broken 3 times in 5 years. Only once i had to get the crank changed as half of the broken bolt was stuck in the crank which cannot be removed.
no nut is fine only the carter pin broke inside the pulley and the pulley did some revolutions with broken carter pin and its grip pins got fully faced off it was wobbling very badly. Yesterday got it changed for Rs 1200 kabuli one. Mechanic has told me that if it happens again than the crank will go to kharadiya for repair depending on damage or change it so far working gr8.
mechanic has told me to get the bolt checked regularly as it can become lose anytime.
Thats why I recommended loctite on it,
Originally its a stretch bolt and needs to be replaced once removed - it wont hold much torque when re-used - buy some blue loctite thread locker and use it generously on the threads of the bolt.
btw - your mechanic telling you to throw the crank away if the bolt broke is a load of bollocks - At ZRS we remove broken bolts on the go - the tools used are called easy outs.. And have replaced about a dozen broken crank bolts for Honda D and B series engines