As you already have the center edge in place (already cut, so to say) just switch to Front view, select the Rectangular Selection tool (make sure to uncheck Only Select Visible in the AM), select the points making up half the body and delete them. Then select the points running around the center of your model (in effect the points making up the red line in your attached image) and make sure they are all on X:0, create a Symmetry object and drop your mesh in that.

/Anders

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