M_Boats: M-15 ballast replacement

Does anyone have information on the best way to replace the iron ballast inthe keel? I discovered a leak coming in on the port side about 6” aft of the join of the keel with the bottom. it is additionally at the first edge of the first “lap”. I’m thinking ballast replacement because drilling into the area produced a lot of rusty water. Additionally, the lap is longitudinaly angled and protruding instead of being a fair curve’ the starboard side is similar but not so pronounced (nor leaking). No problem with raising or lowering the centerboard. Any suggestions would be appreciated