As far as the Fender Made Pickups you have in your Corona Starfire are concerned, some people like them and think they are "good Enough" and some folks don't really like them and feel they can get more out of the guitar with different pickups. The Duncan pickups that Guild offered from Corona in Bluesbirds were not available from the factory in the production Corona Starfire guitars, Just the Fender made HB1's. Personally I fall in the category of not liking them and changing them out. The Corona made Starfires were not routed to be able to accept Original Larger Guild HB-1's (or new HB1's) from the factory, They were routed for standard sized Humbuckers instead.
If you find yourself wanting more out of your guitar than what the Fender made pickups offer you have a few avenues you can explore for a reasonable amount of cash.
1. If you like the look of the Fender Pickups you can always send them out and have them rewound to whatever spec you want. There are several companies that do this for a reasonable price with a relatively quick turn around time. I'm sure a member here has used a service like this in the past and can point you in the right direction if you need it. This option would allow you to retain your original Mounting rings with no modifications but the guitar would be down for the amount of time it takes to get your pickups back.
2. If you decide you want to change the pickups out, your options are really only limited to how much you want to spend on a set of pickups as your originals are standard size and there are 100's of options out there to explore. If you do decide to change out your originals I would suggest getting a new set of pickup rings for a Gibson 335/les paul style guitar (about $10-$15 from any guitar shop or online) as this will allow you to keep the originals with the original pickups. The new rings should mount right up to the original Ring mounting holes in your guitar. Also you don't have to drill the original pickup rings for the one adjustment screw on each side or go through the trouble of making an adapter to use the original rings and if you ever decide to put it back to original to sell you can just remount the original rings and pickups with no permanent modifications to the guitar.
3. As far as Standard sized pickup replacements I have heard that the Seymour Duncan 59's have some of the same qualities of the Original Guild HB1's. I am sure someone here will be able to say more or less if that is true. I have a set of Duncan Custom Shop pickups in one of my Starfires and I love them but they are on the high end of pricing at around $400-$450 a set. I have also used on a Budget GFS Hot Nashville Retrotron pickups and they sounded Amazing in my Starfire III and my Herb Ellis ES175. The GFS Pickups can normally be purchased brand new for around
$70-$90 a set delivered to your front door and they have a large selection for many different tonal options. Every set of GFS pickups I have had I have loved and they are on par with any pickup maker as far as quality and tone plus they are 1/4 of the price. Here is a link to the GFS website with soundclips of their Retrotron series pickups. The soundclips are half way down the page and they have a ton of other pickups to look through as well.
http://www.guitarfetish.com/GFS-Retrotron-Hot-Nashville-Overwound-Humbucker-Chrome_c_120.html
4. One other option would be to modify your guitar to fit the Original or Reproduction Guild HB1's. To do this you will need to have the original pickup routes "Opened Up" on your guitar to fit the bigger HB1's and possibly drill different mounting holes for the bigger pickup rings as well. This is the most invasive and Permanent choice you have as the modifications to the guitar are not reversible. It is an option but I am not sure you can return the guitar to original if you wanted to or if the Original pickup rings would cover the new mounting holes if you tried to return it to original.
Best of luck in your search and personally if I was on a budget I would definitely check out the GFS pickups. Lots of Bang for the buck. Of course as always this is just my opinion and others may differ. Hope you get things rolling in the right direction!
TX