Iderval Da Silva, 46, was punched and kicked by youths in Battersea, west London, in May last year.
He was left unconscious on the ground and died three days later from a bleed on the brain.
Following an Old Bailey trial, a 16-year-old boy and 19-year-old Jadan Richards, from Wandsworth, were found guilty of murder.
A youth aged 17 was found guilty of manslaughter but cleared of murder.
Prosecutor Tony Badenoch QC had told the trial how Brazilian Mr da Silva had parked his scooter outside a Battersea cafe on May 25 last year.
A group of young men spotted the unattended moped and one of them made to snatch it, he said.
When Mr Da Silva tried to stop him, he was set upon with "fists and feet".
Mr Badenoch told jurors: "It was a momentary explosion of short-lived violence which killed Mr Da Silva."
The jury deliberated for more than 20 hours to reach its verdicts.
Jasire Frazer, 18, from Wandsworth, and a 17-year-old were cleared of murder and the alternative charge of manslaughter.
Another 17-year-old boy was cleared on the judge's direction midway through the trial.
Judge Mark Dennis QC adjourned sentencing until February 5.
Sally-Anne Russell, from the Crown Prosecution Service, said: "This is a tragic case of a man who died trying to protect his property. However, faced with the youths he was outnumbered.
"Witnesses watched on in horror as the unprovoked attack unfolded. They rushed to Mr Da Silva's aid after the attackers fled, but the level of violence was such that a few days later Mr Da Silva died from his injuries.
"This was a senseless loss of life. Our thoughts are with the family and friends of Mr Da Silva."