Why not see what PortAsm has to say about it? If it's not self-modifying or counts on instructions being of a certain size, it could be a starting point.
Maybe worth a try but I have my doubts as the asm source uses a lot of <rs> stuff, i.e. nested structures. So question is if PortAsm knows about Devpac/PhxAss syntax (I had to convert it by hand, it took my two weeks to make a perl script for conversion to gas; vasm didn't exist for Atari back then) and if so, if it can handle such a complex stuff (esp. where unsupported floating point sizes come into play). Not an easy task but certainly doable in less than a month.