I definitely wouldn't cheat - it will be obvious and you will be caught out.
What was the reasons for needing to rewrite the first half? Look at those first so you are clear what you need to do.
12000 might sound impossible by September but (assuming that it is due first week of Sept) there are 10 weeks including this week.
So, that's 1200 words per week.
If you work on it 5/7 days that's 240 words per day which isn't much at all and gives you some spare time for checking/editing.
Obviously, you may write more than that in a day but worst case scenario you just have to write 240 per day, 5 days per week.
Start with a plan - work out the main sections and make sure you have all the information you need. Then, start writing - a draft first, don't worry to much about getting perfect sentences etc - do that when editing. You will likely find you can actually write a lot more than 240 words per day.
Can you call in any family/friends to give a hand with childcare or cooking/cleaning? If not, lower your cooking/cleaning standards for a while - no one will die from having soup/beans in roast for tea a couple of days a week.
Housework - just do the basics and catch up once the dissertation is done.
Can you take some holiday from work to spend a couple of solid days doing it?
Good luck OP - you will be able to do it if you break it down.