In this sort of nearly impossible task, one tries any and all sources. A negative, as strange as it sounds, is at this point as valuable as a positive response.
The first problem is not the almost certain unwillingness of Mexican families to discuss out-of-wedlock children, assuming that is what you mean, although that certainly is a great obstacle.
No, the biggest problem is the lack of any real reliable access to any sort of records which show people by name.
I will set a computer alarm, (SET) and when I go to Mexico City in a few weeks, will do the only thing I can: Look up the family name
Asencio in the phone book and see how many there are.
Usually, this does not work. And, the number of years which have passed reduce the odds even further. However, I have several times found people who are still alive, by a variety of tactics.
Um, I might as well mention in most liasons, Mexican men do not give accurate information, though I am only assuming it was this way 60 years ago.