How good a replacement is a rather complex question, mostly revolving around what you are looking to do with it. It looks from the datasheet that the 2n9013 has twice the Ic (continuous collector current) of 500mA, compared to a peak in the 3904 of 200mA. The 3904 is definitely intended to be a signal amplifier, and has a gain bandwidth product of 300MHz. The 9013 datasheet I found doesnt even have GBP, which to me tells me its much more intended as a current buffer, than as an amplifier. The fact that they tote the Hfe (essentially, but not the same as beta) as being linear suggests to me that it expects to have large signals fed into it, and not do much more than buffer an input to an output (hence the need for linear Hfe, but no mention of GBP). The other thing, that line 1W OUTPUT AMPLIFIER OF POTABLE RADIOS IN CLASS B PUSH-PULL OPERATION (I was as shocked to see the word potable is actually what they said on the datasheet. I'm like 90% sure they meant PORTABLE, not that its safe to DRINK the transistor). That line thats "worse than Chinese" means that its intended to be used with its PNP complimentary transistor the 2n9012 on the output stage (hence the class B push pull bit) of a power amplifier.
The long and short of it is this: its a good replacement as long as what you are replacing the 9013 with works the same for you as it did before. That is really all that matters. Generally tho, most designs are not THAT component parameter sensitive that they wont work with the wrong VAGUELY similar part.
Good luck!
