I totally agree with Steve and Andy.....you should use any assembler for the 8051 that is available to you. The beauty of assembler language is that there is a 1::1 correspondence between what the original author wrote/assembled and what you can expect to get as you assemble with another tool set.

There would be another big benefit in adapting any peculiarities of the original source code to the alternate assembler in that it would force you to comb through the code and understand its structure.
This can be of huge value for two reasons....

1) You are miles ahead if you should need to get into special debugging in the case that the code does not run correctly after your assembly process.

2) Assembling old code is almost always set to imply a substitute MCU or additional features and so familiarity is necessary anyway to be expert at making any necessary changes or adaptations.

Michael Karas