Lost Crypto keys can be difficult, if not impossible to get back. The Crypto keys are critical to be able to access these assets, and the best option may be to simply write these down, ideally twice for the sake of redundancy, and to keep them in two separate locations such as a safe deposit box and a home safe. Backup USB drives are also a good idea, but these can fail.
You should also consult your Estate Planning Attorney about the income tax benefits of a “stepped up” tax basis associated with these assets and ensure that the basis of said assets are tracked and recorded appropriately. Stay tuned for more Crypto Estate Planning updates as they arise.