Tools:
IDA Pro 5 (Freeware) is the last Freeware version able to handle DOS executeables (official available by the ScummVM devs https://www.scummvm.org/news/20180331/) - IDA Pro still supports DOS-stuff with latest release - but not the freeware
or Ghidra - but the DOS/16bit support is sometimes lacky - but the decompiler is builtin