Macro Assembler AS accepts a wide variety of syntax, so the first step I would take is to switch to using that and then start looking at what options you'll need to set to make the existing source assemble with as few errors as possible. You'll want to start with a CPU 68000 directive (or similar) at the top of the file and then look at the errors. If you're getting errors about the hexadecimal number format because your code is, e.g. using $ABCD and the standard syntax for the CPU wants 0ABCDh, RELAXED ON will usually fix that. If the code isn't using DC[.Size] and DS[.Size], you can define macros for whatever your source code happens to use. And so on. Most labeling systems should work just fine; it supports most of the common forms of implicit local labels (.foo, +, --, etc.) and has a [url]=http://john.ccac.rwth-aachen.de:8000/as/as_EN.html#index_SECTION_1SECTION[/url] directive for explicit local scopes.

The only thing that AS really insists upon is that only labels start in the first column and that instructions and assembler directives never start in the first column. So some assembly formats that have indented labels (with the label always followed by a colon) and the like will need manual editing.