You're not going to get audited unless we're talking about very large amounts of money/Bitcoin. Go back in time think about when you bought look at where price of BTC was when you bought and do your best. Even in the rare case you are audited you more than likely won't incur any penalties or fees unless they sense deception or like you were hiding something. If you get audited tell them the truth and you tried your best and you made the mistake of not keeping records and you'll be fine. You'll just have to pay any remaining tax you may have missed. dead serious bro.