I purchased MS Flight when it was released, but have since lost the file.

Is it possible to redownload or repurchase the sim?

I hope you didn't buy the base game, because it's free. You can download it here: http://go.microsoft.com/?linkid=9800616

 

If you purchased any addon content, you can download that content via Steve's (Stonelance) Flight Toolkit, which you can find more information on in this forum and at http://flighttoolkit.com.