You don't want to repair a busted spring. Replace it. You can buy an assortment of leaves like that one for a few dollars or you can buy complete, assembled leaves with a long rod attached that you simply cut and bend to length. Any of the majors - SLarose, Timesavers, Merritts, etc. have them.
Length and thickness of leaf are not critical although they will affect the speed of regulation. If pendulum length is not critical - ample space - then just select one about the same length and thickness and commence regulation.