Hi!
I have a lot experience in DOS apps conversion to modern operating systems, but in your case I suggest different solution: use any open-source dos box emulating virtual machines with some code modifications to support data files input and output and additional command-line or ActiveX interface to support your needs. This way give us a little bit less performance in data files processing, but ensure normal work of original algorithm - it works for years in existing exe file and already tested. Trying to decompile exe, restore original algorithm and recode it on modern language possible, but take a lot of time, cost much more than you expects and take a relatively long time to test and find all bugs appears due to decompilation nuances.
Fell free to ask any questions. I'm always open for discussion.