There is a simple solution. The only thing with enough energy to recharge our magnetic field is about a million bolts of lightning. Just put a couple hundred turns of 000 copper wire around the equator, ground one end and stick the other end up a couple thousand feet into the air and wait for a few thunderstorms to pass by. Don't wind the wire in the wrong direction or you'll degauss the whole planet instead!
Viola!
Woody
You know, that sounds like it just might work.
I know you can magnetize a screwdriver by wrapping copper wire around it and connect it to a battery. Good thinking.