Any regular potting soil would probably be ok, the key is sterilization. You can nuke it in the microwave to easily do this (I do this occasionally for seed or cutting starting). Just use a microwave safe dish & nuke for about 3 minutes. Then keep coverered while it cools enough to use. I generally just use a gallon size ziplock bag - leave it partially open while nuking, and then close it after you remove it for cooling. You might have to do this several times to get enough soil to repot your ficus.
I've read that fungus gnat larva only live in the top inch or so of soil. You could remove just the top incho or so and replace that with sterilized soil. In addition you can add an inch or so of sand or gravel to help deter gnats from continuing to lay eggs in the soil. Fungus gnats only live a few days, so killing the larva and inhibiting egg laying should take care of the issue.
