The car, originally modified by a 12-year-old Brock with an axe, was the vehicle in which Brock taught himself to drive on the family farm in Victoria.
"It's truly amazing," Brock's brother Lewis said yesterday.
"Peter drove it everywhere around the farm with me sitting on the back, most times holding a battery.
"He caught the motor sport bug in that car.
"It's where he learned his early race craft.
"The thing had no brakes so Peter would have to throw a huge slide just to stop it."
Brock died in a rally crash in Western Australia in September and a national search for all his cars failed to unearth the original.
The modified car is believed to have been sold by Brock's father, Geoff, along with other junk when cleaning up the farm.
The chassis was found "stored" in the roof of a factory in Victoria last month and it was identified by young Brock's axe marks.
The car was bought from the factory owner and will be presented to the Peter Brock Foundation.
The chassis will be fully restored to original condition with the help of the Austin 7 club with a view to featuring at future historic race meetings.
The car was originally purchased as a road car by Brock's father before being modified by the axe.
Father and son then welded a steel frame to the chassis and installed a seat to make Brock's first racer.
"It's a miracle it has survived," Lewis Brock said.
"It was a bit like a 1950s go-kart.
"It was instrumental in him realising he had such an affinity with cars, racing and driving. It was paramount to him deciding on racing as a career.
"It's the first car Peter built and the first car he drove. It's very significant to his history."