I am not asking about the likeliness of this scenario happening. I only hope that it is realistic enough to influence the thinking of politicians today and that as such this question doesn't fall under absurd hypotheticals, which would be off topic.
On the UK side, the biggest target is the proposed European Union (Withdrawal Agreement) Bill, which would be intended (among many other things) to "undo" the effects of the European Union (Withdrawal) Act 2018 such that EU law (and changes to it) continues to operate during the transition period named in the withdrawal agreement, and to provide mechanisms for parliamentary scrutiny of ongoing changes.
In principle, the original Withdrawal Act provides options for Ministers to make any necessary changes via "Henry the VIIIth powers" and statutory instruments (that is, effectively writing a letter saying what the new laws are). However a suitably upset House could object to this behaviour and lead to an awkward mess where a) EU law doesn't apply when it should b) It applies when it shouldn't or c) no-one knowing what the law is. In all cases one can expect people to look around for someone to sue and repeats of the kind of mistakes which have already cost the UK government millions of pounds.