Well, if you really want to go the difficult route....
Here's is a ZIP file containing a disk image that includes Astrobirds:
Unzip that file anywhere you like.
In Atomulator you then need to:
1. Go to Settings / RamRom and uncheck RAM/ROM DiskROM Enabled. This disables the AtoMMC ROM and enables the original DOS ROM.
2. Go to Disc / Load Disc :0/2 and choose games1.dsk
3. On the keyboard:
The default keyboard mapping reflects the positions of the keys on a real atom, as far as is possible. This unfortunately makes Astrobirds rather unplayable.
You can find a collection of correctly fomatted disk images here:
But honestly, using the Atom Software Archive is much easier!