I looked for a year for a x39 Handi and I couldn't find one either.
I got a Rossi x39 and it's good gun. I have not rechambered mine because I specifically wanted a x39.
The barrel is an extractor. As far as accurizing... I haven't had to do anything to it, I put a 4x Weaver on it and started shooting. Rossi rifles come with an O ring on the forearm stud straight from the factory. My gun will shoot about 2.5" group at 100 yds with surplus ammo. It might do better with quality ammo.

I know the Rossi's sometimes get a bad rap but honestly mine is a good gun and it shoots well. I have no complaints with it. I would think a rechamber to either the 303 or x54 would be pretty straight forward and would make for a nice shooting gun. One other thing is my rifle is very well balanced and carries well.
I wouldn't hesitate to buy another one.
Spanky