As some once said "ee's avin a larf". I think he should give you the old crown back + the cash, then you can take it to a decent watchmaker. There are a few Tissot crowns on Fleabay at the moment, you may get lucky but there are many, many different crowns. Unfortunately Tissot is part of the dark side (commonly known as the Swatch Group - see other posts) therefore parts, diagrams, sizing, info etc is a bit of a problem.