I have modded many watches. The owners do the leg work in finding out what fits, and what don't, then they source the parts, and send me their watches, and parts to mod. Its not the watchmakers job to find out compatibility and source the parts, this is the owners responsibility.
That said, I have undertaken some modding where I have been asked to do EVERYTHING, including the above mentioned, and obviously this time and effort is charged on the final price quoted.