Does the bike start/idle/run at all on the stand with the rear wheel in the air? Does the bike die when you actually apply the brakes, or when the rear wheel stops moving?
If the bike runs and the back wheel spins while on the kickstand but dies when the back wheel is stopped, you’ve got signs of textbook Garelli clutch issues. Take off the giant circlip that clips on those nine pegs and the whole clutch assembly should right apart. Inspect the small rubber bushing that go on 6 of the pegs and check for signs of melting. Also inspect the entire big rubber clutch itself and all inside the metal clutch drum.
Every Garelli I have ever owned has swelling or uneven wear at the deepest point in the clutch bell. A little friction and improper oil causes the rubber clutch to melt and smear onto the metal drum. There’s usually bits of melted rubber all over in there, including the small oil holes and behind the clutch bell and gears.