Well, to be fair the the FFG writers that worked on Escape from Mos Shutta and Long Arm of the Hutt, the Krayt Fang was really meant for a beginner game rather than the full ruleset, and so not a lot of concern was given to how many hard points were left or specifically what modifications were made. It was probably "here's the base YT-1300 stats, let's just tweak the numbers a bit." And I doubt you'll ever see the ship get mentioned ever again.
As HappyDaze noted, the stats do indicate a High-Output Ion Turbine (nice catch on that one), so it's not impossible to reverse-engineer the Krayt Fang to determine how many hard points are left.
As for whether the YT-1300 should have two weapons or just the one... I'm thinking that two weapons as default works, largely as the one YT-1300 most Star Wars fans are familiar with has been repeatedly shown to have the ventral and dorsal weapons. But at the same time, if you want to be more of a purist to the older game systems, it's easy enough to strip the dorsal turret and simply refund the YT-1300 the hard point used by the weapon.