I don't think the Baretta 92 series locking block will work, you'd do best looking for a Barretta 51. Of course that will be as difficult as finding a Helwan part.
Your best hope may be, other than online, is going to gun shows. You might find some help here: http://forums.gunboards.com/forumdisplay.php?87-Middle-Eastern-Firearms-And-Militaria
I sold a Helwan at a gun show several months back, I didn't want a gun where parts were nearly impossible to find. Numrich has some parts, but no locking blocks. Also, ProMag does make a magazine for the Helwan.
I bought the gun for $50 and sold it for $250.