This article may be behind a paywall, but the upshot is that coming later this year, Fidelity will allow Bitcoin in 401Ks they administer, likely with other crypto options to follow. Whether your 401 will allow it will partially depend on your employer. Not sure how this will play out with solo 401s.
https://www.wsj.com/articles/fidelity-to-allow-retirement-savers-to-put-bitcoin-in-401-k-accounts-11650945661?mod=latest_headlines
Kevin Slater