I had to do some work to get the source to assemble, but all of the changes were not of a type to substantially affect the program's internal logic or structure. I've attempted to test the implementation. I'm afraid that wasn't very extensive since I'm not a FORTH programmer by any means. Still, everything I tried worked as I expected, based on a couple of books on FORTH I own.
There are no documents for this version, other than a short demonstration program (DEMO.TXT on the DISK1 file). To try and alleviate the large set of unknowns here, I've gone through the assembler source - all of it - and located all of the FORTH "words" that this version offers. Some of them may not be intended for use by the programmer, but I can't be certain. What you can do with this list is when working with the program you can check to see if the word you're looking for is there. That implies you have some knowledge about FORTH, at least have a tutorial or a book handy.
With all of that in mind, here's the list of words in the supplied 6809 version of FIG-FORTH: