I would contact a tax professional in your jurisdiction for assistance. They may contact the relevant tax authorities to negotiate an mutually acceptable solution to the lost records. This should serve as an important reminder to never rely on third party providers to store financial information that is needed for tax purposes. Instead one should store electronic financial records on computers under one's control, running Free Libre Open Source Software (such as GNU/Linux), with multiple redundant backups and in a format that is readable by Free Libre Open Source Software.

By the way I expect a fair amount of grief among former MTGox clients over lost / inaccessible records as the various tax deadlines around the world approach April 30th (Canada), April 15th (United States) etc.