OAuth is the same for both Kraken and Helix (just some different scopes). A token that works with one will work for the other if the scopes are correct.
Where as in Helix you would set the Authorization header to OAuth <Your Token>, in Helix you would instead send Bearer <Your Token>.
One thing worth noting, while legacy apps (such as those from 5 years ago) create OAuth tokens without an expiration, at some point in the future even those tokens will expire and those legacy apps will have the same expiring tokens as all current apps, so if you no longer have access to your account/app then in the future when the token expires you may not be able to obtain new ones.