MTD Tiller with 5HP Briggs and Stratton motor?

We have an 11 yr. old MTD Tiller model 340 with a 5HP Briggs and Stratton motor. Recently there was a flood and the motor was nearly completely covered with the muddy water for nearly 36 hours, then we had sub freezing temps. We emptied the gas tank, have changed the oil 3 times (still a little muddy mirky), and changed the spark plug, yet it will not stay running. We have used carburetor cleaner and it will run as long as the cleaner lasts, then it dies. I really like this tiller and don't want to buy a new one. Any ideas as to what to do. Don't want to spend a lot of money to fix it, yet don't want to spend money for a new one.