You may have to port the assembly to a different assembler, perhaps jwasm (fork of wasm, the Open Watcom assembler) which IIRC is fairly compatible to masm.
I was wondering if masm386.exe is just masm.exe renamed? MASM 6.x supported .486 and .586 in (at least in DOS).
Have you used masm2alp? And alp?
IBM's Assembly Language Processor might be interesting to look at. I see it comes with VisualAge 4, as well as in several standalone forms.
I see also in my searching for masm386.exe that IBM's Macro Assembler back in 1987 included CodeView, and some of their ALP packages included a "cv.exe" as well (don't know if it was CodeView or not, but probably so based on file size).