You can fix his memory if you have a sufficiently high computer skill. And yes, it's silly that you can't use another party member with a high skill for the effort. It's not as though HK-47's inards are keyed to your fingerprints.
But you really gain nothing that you won't receive as soon as your character discovers his true identity. Then, HK-47's own memory access is automatically restored, and his functions as an assassin droid improve dramatically.
To the Righteous belong the fruits of violent victory. The rest of us will have to settle for warm friends, warm lovers, and a wink from a quietly supportive universe.