Note that "deal" will have to replace the existing decades' worth of detailed regulation argued over by many government workers ad infinitum, on a thousand topics. And the treaty says that such a deal is only accepted by a unanymous vote of all EU members.
If the UK leaves this decade, it will be without any deal in place.
"As of date X, all existing EU regulations remain in force. Changes to UK law that overlap with existing areas of EU regulation will result in a notification to the Commission".
Done. There is no need to 'replace' detailed regulation on a thousand topics when, as you observe, it is already implemented in UK law.
If the UK leaves this decade, it will be without any deal in place.