The Holley carb is an antique, and is shared with all sorts of Ford and other US cars.  International Harvester too.  There are various problems with this carb in its Judson incarnation (there are several variations of this carb).  The MGA Judson one is Ford application, List #2527

The detailed manual for this carb is available online here.  Lots of technical tips here.

The plastic cam for the accelerator pump breaks easily, these can be bought from ebay seller auction2200.  There is also a 1904 with a different metal cam, but these are not interchangeable as the actuator is also different.  Both the carbs I have have plastic cams.

The Economizer can be fixed by these guys - if it is not correct (and they usually are not on a rebuild) the performance of the carb will be dreadful.

Air filters available here.  You need the one-barrel variety.

Exploded diagrams here, here, and here.

I gave up on the Holley because it is impossible to set up for the MGA Judson and have correct mixture across the power range.  See this PDF for how to convert to an SU

The first three photos below are of my carb as it was being rebuilt by Michael Mayben.  The jets I chose was a 64 but his comments were:

I did replace the jet with a NEW #52 for testing purposes in order to see how it ran, not so good!  So I have included an assortment of jets in the bag for use in final tuning...I'm suggesting that you use a #58 minimum for beginning your jetting testing.  I "think" you will end up at around a #62 or #64 for your application.