Is repairing and using a found car illegal?

I found a rusted, dented truck in a forest with a missing battery and tailgate. Would it be illegal if I put a battery in it and repaired whatever else needed to be repaired, could I take it as my own? Is that legal?

It depends. You need to check out who owns the forest. If it’s state/provincial land then you probably won’t be allowed to take anything from there. If it’s private property, it’s just illegal. The truck might be there for a reason, so I wouldn’t recommend this.