Ooh no, you don't need a bivvy heater.
A decent sleeping bag for night, and good clothing for day.
Too many 'accidents' with bivvy heaters, burnt down bivvies, and deaths from carbon monoxide poisoning.
Even using a stove in the bivvy is a risk.
Years ago a mate I used to fish with had a run while his stove was on to keep him warm, he only burnt a hole in his sleeping bag, but other mates have destroyed their whole bivvies and woken up in A&E.