get as much out as you can, then add fresh fuel and for the most part it should be fine. Big concern will be if it rusted the inside of the tank or not. I have seen some get so bad that the brass float was all eaten away. You can run it short term with rust in the tank, but you will be plugging the filter fairly quickly. Easy way to see it is to drain some fuel after it is run a little, and if it appears kinda orange, you have rust bad.