How can a gangster be portrayed as a positive role model?  What are
some examples of this in film? How could Tony Montana from Scarface be
considered one of these positive role models?

You'll need to focus on the positive aspects of these individuals
rather than the negative to see how their 'Characters' could have a
positive or inspiring influence on someone.

Its about showing hardships that are overcome. Which each and everyone
of us experience in our lives daily, and struggle with coming out on
top. Even in our small lives.

Showing the extremes of life, which most of us do not experience, and
putting a positive spin to give us inspiration.

The portrayal of gangsters in cinema and television has romanticized
and glorified their "code of honor."  The gangster is not a murderer
and a criminal, but rather a "soldier" performing his sworn duty to
his superior.  (Lieutenant, capo, etc.)  These films emphasize
brotherhood and La Costa Nostra's oath of Omerta.  Coupled with this,
they show primarily the acts and methods of the mob used only in
situations we as the audience can relate to:  a neighborhood girl gets
raped, and the police let her attacker off on a technnicality.  A
friend's car is stolen, and no one can locate it.  Law enforcement is
portrayed as bound by rules and regulations, powerless to stop the
offense; the soldier is free of these obligations, and capable of
cutting through the red tape to provide justice.

Primarily, the strongest tool used to portray a mafioso as a positive
character is point-of-view.  Hero, anti-hero, or villian, an audience
will root for the character they know the history behind, unless the
director gives them ample reason not to.  (A common shorthand for this
in film is kiling an innocent woman or a child.)

Does this answer your question, or is there a more specific aspect of
this portrayal you wished investigated?