I've seen cars in lakes and swimming pools, but a pond is a first for me also.
If I remember correctly, rock is readily available and inexpensive in your area, so I would replace the rock. If this creates a budgetary problem then, depending on the degree of contamination, the existing rock could be cleaned using a power washer and de-greaser. Some hydrocarbons may remain, but probably not at a level that would cause a problem as it would be picked up by the skimmer and filter pads. These in turn could be cleaned with Dawn. (Yep, the stuff actually works.) Without seeing the level of contamination I can't really advise you either way. It will be an educated judgement call for you.
"Everybody is ignorant, only on different subjects." - Will Rogers