I have recently done such a project for a Client.
IMO, and with due respect to yourself, I think that whoever assigned the task might be somewhat misguided in choosing a self-confessed assembler novice (unless this is specifically intended as a learning experience to enhance your assembler skills).

In order to convert from any language to another language you need to understand both languages and what the code is trying to do!

So, if you're new to assembler, your first step is probably to get to know assembler.

If you're familiar with assemblers in general, but not 8051 assembler in particular, you should just need to study the 8051 Instruction Set & the manual for your assembler.
The Philips databook has a good section:
80C51 Family Programmer's Guide and Instruction Set manual:
http://www.semiconductors.philips.com/acrobat/v...UIDE_1.pdf

How well documented is the code?
Good documentation will help, but beware that real code often gets out of step with its documentation; if they disagree, you will need to look closely at the code to understand what's really happening - which brings us back to a good understanding of assembler.