You can fight back the grin that wants to creep onto your face, every so often, during “In Like Flynn,” the good-natured, not-totally far-fetched Aussie film based on the early life of Australia’s first and greatest screen icon — the swashbuckling rake, Errol Flynn.

It’s a patchwork affair — with four credited screenwriters, including a “Flynn.”

There’s not a lot of star power in the cast and Aussie director Russell Mulcahy’s big-screen career went off the rails with “The Shadow” and “The Real McCoy” nearly 30 years ago.

But Mulcahy, who did a few “Highlander” pictures and a “Resident Evil,” knows how to shoot action. And as long as “In Like Flynn” is playing up the two-fisted, devil-may-care bravado of its subject, it’s on solid ground — brawling, dodging straight razors, daggers and bullets in the days before he “went Hollywood.”

That Hollywood trek could have happened earlier than it did, according to this “mostly true” movie based on Flynn’s memoir, “Beam Ends.” We meet him in the bush of Papua New Guinea in 1930, leading a Hollywood producer (Dan Fogler) on a trek to secure grisly footage of the victims of the primitive natives fighting off a new gold rush, and the intruders that come with it.

The politically correct Flynn (Thomas Cocquerel of “Kidnapping Mr. Heineken”), all of 21, may not be as seasoned as he makes out. But as he fires warning shots into the air rather than at the murderous tribespeople, dodges arrows and a hail of blow darts (which kill a native guide and friend), crossing a croc-infested river to make his Indiana Jones getaway, he shows manliness and star quality to that producer.

“You find yourself in Hollywood, come find us.”

But Flynn has that gold on his mind. He recruits a couple of pals — the tough, hard-drinking cynic Rex (Corey Large) and pedantic, posh-accented Dook (William Moseley) — to join him. They outfight/outwit Achun (Grace Huang, venomously sexy), the Dragon Lady of the Sydney waterfront in stealing her boat — the first Flynn sailing schooner to be named Sirocco (“desert wind”) — and make their way up the wild and wooly coast of Oz.

The previous owner of the boat, a weepy, violent old salt named Charlie (Clive Standen of TV’s “Vikings” and “Taken”), muscles his way into their partnership.

There are prize fights and shootouts when they aren’t threatened with dying of thirst, starvation or drowning by sinking.

Death hangs over the story and marks Flynn’s life, this version of his biography tells us. “Beam Ends,” the sailing slang title of that book, hints at despair, desperation and life of narrow escapes that Flynn says he was living at the time. “Beam Ends” means a boat that’s heeled so far over in a strong breeze that it’s about to capsize.

As we know he got his start in Australian films shortly after this period and that he wasn’t a literal Hollywood “discovery” (“He’s a headliner and doesn’t even know it!”), it’s hard to say where the truth ends and the Flynn memoir and four-screenwriters-adapting-it fiction begins.

The good parts of “In Like Flynn” make you wish there were more of them, and that the movie’s modest budget and modest ambitions weren’t slowing it down, thanks to a screenplay that serves up dead stretches in the midst of action picture cliches.

Digitally recreating Sydney Harbor in 1930 is fine. Digitally adding dolphins to the Sirocco’s wake? Less so.

The Indy opening sequence is quickly topped by a short, riotous, dodge the razors, punch the thugs and kiss the girl brawl that gave me high hopes for the picture. Dropping in on the opium den/brothel HQ of Achun didn’t dim those hopes. Much.

The idealistic young Flynn crows about relishing the moment, as “We will never be these men again,” and expectations rise.

“The sirens of the sea beckon!”

And then the movie unfolds in an updating of the corniest, old-fashioned two-fisted balderdash that Flynn’s movies, way back when, used to be.

Only less fun.

Standen has the best lines and the best role, warning the lubbers on board his boat — “Thievin’ sissies” he calls them — to brace for “Cyclones that’ll blow your foreskins off.”

“Won’t be long before y’get yourselves dead.”

But as the young Flynn revels in the low-rent adventure he’s dragging them all into, the old salt makes him consider his lack of ambition and the future (which Charlie is sadly living).

“You need to hoist your sail a little higher, mate.”

The handsome Cocquerel has an athletic grace and just a hint of the bravado that suits the role, and it’s no surprise to see him standing in the bosun’s chair on the top of one of the schooner’s masts. Young, relatively unknown guys are the easiest to talk into doing their own stunts. But he’s merely adequate in the role, with nothing that suggests Flynn’s brash physical presence or the wicked glint Flynn brought to his camera-loving grin.

Moseley manages a few moments of comic relief. What sort of ‘bath house’ is this?” Dook asks those in the Townsville (Queensland) brothel they’ve ducked into.

“The good kind,” a lady of the evening informs him.

David Wenham (“300”) plays the crooked priest/mayor of Townsville, Callan Mulvey the not-nearly-writerly-enough writer marooned there, and Isabel Lucas of TV’s “MacGyver” is a cunning, brawling hooker/ex-girlfriend of Flynn’s, sort of the Karen Allen to Harrison Ford’s Indiana Jones in this picture.

“In Like Flynn” would probably benefit from lowered “cut-rate Indiana Jones” expectations. But Mulcahy is too visual (a music video vet) and visceral a director to not lift them, just a bit, in the best of those early scenes, before the weary screenplay limited supply of charisma in the cast let him and the movie down.

MPAA Rating: R for some violence, drug use and a brief sexual reference

Cast: Thomas Cocquerel, Corey Large, Costas Mandylor, Isabel Lucas, William Moseley, Grace Huang, Nathalie Kelley, David Wenham, Dan Fogler and Clive Standen

Credits: Russell Mulcahy, script by Marc Furmie, Steve M. Albert, Luke Flynn, Corey Large, based on the Errol Flynn memoir. A Blue Fox release.

Running time: 1:46