If it ain't broken don't mend it. As Erik has said testing alone will be a major task in itself. A good design with well commented code in most languages is understandable by another competent person. So first be clear what and why you want do it, and do coding of precisely those parts of the design that is as Andy says interface to C. It may be so that the asm code is poorly commented/ documented but working well, so just good documentation will do the job.