(1)Melt out all the remaining lead with a plumbers torch. Do not sand or grind the lead because of lung issues.
(2) sand blast the rusty area, then wash with Dupont #3812 enamel reducer.
(3) Fill the area with a fiberglass based body filler. "MarGlass" by Evercoat is one to consider.
(4) Top coat with regular body filler if required.
(5) Prime the area with Dupont Variprime etching primer, then spray with 2K high Build primer or whatever other product you like to use.