First you need to get the first barn find which will get you the 1932 Ford De Luxe Coup which is required, So go to custom upgrades, then select conversion, then select engine swap and buy the S1 Class 5.2L V8 engine now auto upgrade your vehicle, now you are ready to race. Now just go win any S1 Class campaign race and the achievement is yours
tips from comment sections
For the event I'd recommend the "Half-Pipe Climb" circuit at the bottom left of the map. Takes a minute to complete when you put the number of laps to one, and the half pipe is great for overtaking.
Posted by Wyyvern
I won every race in a few S1 championships with the car and it didn't unlock. I had to do just a regular campaign race to unlock the achievement.
Posted by v SATAN x
Don't autoupgrade first, do the engine swap first. If you autoupgrade first you are paying for engine parts on the engine you eventually replace.
Posted by SatNiteEduardo