If I still have all the keys is it possible to reactivate a stake pool that was previously retired?
All you need is the cold key to reactivate a stake pool. The rest can be replaced. If you have all the keys and you want to reuse them, you can. But only the cold skey matters. I assume you will need to pay attention to the counter for the operational certificate. You will need to have a bigger counter (preferably +1) for the operational certificate than the one used to mint the last block.