Yes, you can use old pushrods as long as they are not bent. It is important for the pushrod to be straight. There is some concern about how the pushrods wear, so if you are going to reuse old pushrods, it is best to make sure you install them exactly as they were removed. Some will say this does not matter, and in most cases it does not. It is true that pushrods do wear and if you are being extra careful, putting them back exactly as they were removed is a good practice.