I remember my friend once asked me what would fem Shinichiro's name be? In fact it's very simple. Shinichiro's name means "true/new first son." In the feminine form "son" would change to "daughter." So probably her name would be Shinichiko.
Edit for this line: I also like the version Shiniko. I looked online and Shinika isn't an official name in Japan lol. But! The form Shiniko can be used simply as a cute shortening of the name Shinichiko itself
But this is if you want to preserve the orig meaning of the name. Traditionally in Japan girls are rarely given names with numbers. Most likely if fem Shinichiro was named traditionally(!!) she would be Hatsuko (the most classic name for a first daughter) or Kazuko/Ichiko (literally means "first daughter"/"first child")












