The calculation dialog should have the same Find/Replace tool that exists in the menu for doing find & replace of data. The engineering for text find & replace is already existing in FileMaker. Nothing new needs to be engineered to make it perform the same thing inside the calculation dialog. (Maybe Script Workspace too, for editing script steps, but I realize that would require new error detection, whereas the calculation dialog already detects errors when you close it)
Yes, there are plug-ins that offer this feature, but it would be more helpful to have it included in the FileMaker Advanced application. Not Pro.