And without tools, it is the safest way to achieve this?
Assuming the code was written "on-site" find the original specifications and recode.
If the original specs are not to be found, Observe the actions of the code (depending on the functionality/comnplexity of the module, this may take considerable time) and write new specifications. Then re-code.
If you have hundreds or thousands of missing source modules investing in a "tool" might be worth it. For one or 2, just re-implement.
If you re-code, there is probably no need to re-write in assembler. . . Unless you happen to have considerable assembler expertise available.