They actually did that Engine. Very often, doors, fenders or other pieces from other cars were put togeter to make one screen worthy car again. There are many different versions of how many cars were actually wrecked. some cast members suggest around 250. Stunt men say over 300. Either way, it's a lot of cars.
The most recent stat I saw was a little over 200. The last season there were only a half dozen General Lees left on the set due to anything more dangerous than having the Dukes climb in and out of it being done by 1/8th scale models of the GL. Not a real car. The 5th season was supposed to be one of the more brutal season since they were killing chargers in wholesale fashion trying to make up for "Bo & Luke" being on strike. So they killed a lot of cars to ratings up in their absence.