If the paint is that old then it should be pretty stable by now! You should have no problems with any enamel paint and certainly not the Humbrol enamels. You could probably get away with acrylics too but I would advise your Dad to stick to enamel.
If the hull after rubbing down looks a bit patchy then he could give it a coat of the grey primer before applying the topcoats for an even finish.
Let us see a picture when the job’s done.
Colin