I've found G1 factory curls surprisingly durable. If the frizz is not extreme, simply washing and conditioning them may help. My personal standard method is to comb through the dry hair to untangle anything you can. Then shampoo and condition the hair in cold water, gently squeezing the curls in your hands, and they will spring back to place. Dry on a horizontal surface.
I've got no special advice on the Megan doll, as I've never defrizzed one.