There are a few things you could try.
Take a sample and mix pre-determined amounts of beer with water
When it gets to tasting good try watering down a gallon and force carbing it.
If that is good then do the whole batch.
Pre boiled/cooled water of course
I had this happen with a Wee Heavy and 90/- parti-gyle.
Used my HLT as an electric mash tun and forgot about the high set point (moron I know)
The Wee Heavy I soured and will know in a year if its any good.
The 90/- is going to be an 80/- this weekend