Swapping magnets is probably the easiest and cheapest mod you can do.
Also, one cheap possibility is sourcing Gibby 498T, people seem to hate on that one, but it's actually very closely voiced to the Custom line of the Duncan, in fact it's in between C5 and regular Custom, but it's closer to C5, except it's not as clean/precise and it has more mids to my ears. If you look hard enough, you can find one for a decent money - more then a magnet, sure, but still a steal for a Gibby pickup. And if you like Custom, chances are you'll like 498T as well.
Used to be a time when one could find these for 30 bucks, back when Gibby had putting them in almost every guitar they were selling...