An Arlington jury convicted Alan J. Arnone, a 24-year-old Frostburg, Md., man, of involuntary manslaughter yesterday in the beating that left a Washington political campaign worker dead near the Iwo Jima Memorial in the fall of 1976.

The jury recommended that Arnone, who was being tried on a charge of murder, be given 12 months in a vocational rehabiliation facility and be fined $1,000 for his part in Ronald J. Pettine's death.

Under Virginia law, it will be up to Circuit Court Judge William L. Winston to sentence Arnone, one of three men accused of the crime. The judge, just a week before the three-day Trial began, rejected a peal bargain agreement that would have resulted in a similar sentence.

One member of the five-man, seven-woman jury said some jurors were fearful that the 4-foot, 9-inch Arnone, who suffers from multiple birth defects, "would never make it in prison . . . with his physical and mental abilities.

"It was a real struggle," said the juror, who asked not to be named. "We knew he was guilty, but the question was of what and to what degree." The jury "came close to being hung because four or five members wanted a harsher sentence," the juror said.

The jury reached its verdict after seven hours of deliberations over two days. The action brought to an end the trials of three men who ware charged with beating Pettine to death in the piney woods near the memorial.

Although convicting Arnone of involuntary manslaughter, which can carry a maximum of five years in prison, the jury found him innocent of a charge of robbing Pettine, who worked tin the presidential campaign of Rep. Morris Udall (D-Ariz.).

Arnone initially appeared dazed and confused by the verdict, but he broke into a smile after he was congratulated by members of his family. "It was the best thing that ever happened to him," said a sobbing Michael Arnone, Alan's father.

During the trial Alan Arnone admitted kicking Pettine during the beating that led to his death. But Arnone's lawyer insisted during arguments to the jury that Arnone was the unwitting dupe of two other men convicted earlier for the death.

The other men, Michael G. Simoneau, 18, of Falls Church, and Charles A. Bamman, 22, of Arlington, are serving long prison sentences for first-degree murder in the slaying.

Arnone, who is free on bond pending his sentencing June 1, was working at a restaurant in Northern Virginia at the time of Pettine's death.

Arlington prosecutor Kenneth Melson said he was "not at all upset with the verdict. From a justice point of view, justice was vindicated."

Melson said that prior to the trial, his office had been considering vocational programs for Arnone. "The verdict goes along with what we were all thinking," he said.

Arnone, was portrayed by his attorney, William D. Dolan, during the trial as a "follower, a tag-along . . . a mascot for Simoneau and Bammon."

Arnone told the jury the two men had sexually abused him and on the night of the slaying-Oct. 2, 1976- Bamman "yelled at me and told me to come and kick [Pettine] I was afraid of what Chuckie [Bamman] might do to me. He said, 'If you don't kick him, I'll kill you.' "

Arnone said he kicked Pettine because he feared his companions would make good on that treat. In previous court testimony, one of the three defendants, said the men went to the memorial, a known night-time meeting place for homosexuals, "to smack around a few queers."

Pettine married and the father of two children, was described in court as a homosexual. He had left a wooded area near the memorial when he encountered the men, according to testimony.