Not a handheld, more like a 'tv game'.
Did you make pcb photos? (chip labels, XTAL).

According to that emulator, CPU is Z80 @ 11059200/4.
It is GPL2, but this hw looks simple enough that you wouldn't have to copy paste source code.

Astro Thunder/Firefox: Years ago, Kevtris tried to dump it but couldn't figure it out. It's a uCOM-75(unemulated), and also a 2nd MCU for sound.