You will certainly get benefits from your long term fast, whether you still need the operation afterwards or not. You will be stronger for the operation and no doubt will get healing that wasn't 'expected'. It may be possible that your body can heal itself enough for you to not need the operation at all also. It is a chance I would certainly take.
Whether you need to fast for 90 days or longer depends on your stores though. As a rule only those who are well overweight to start with, or who aren't but have fasted before and have very good quality stores are likely to be able to fast this long safely.
In either case though professional opinions should be sought.
On this page you will find experienced fasting supervisers and doctors willing to do consultations by phone or online in different formats:
On this page is a list of fasting retreats where you can actually go and be supervised through your fast if you can afford it. (It should be cheaper than the operation but maybe not.) Some of them will also no doubt be able to offer consultations also.