There are about 135 games in the category "sci-fi arcade topdown scroller":
http://www.mobygames.com/browse/game.../top-down/dos/
*) vertical or horizontal?
*) powerups across the map, or collect gold to upgrade your ship at the merchant between each level?
*) ega or vga graphics?
By the way, I recently got to addicted to a sidescroller remake too, called Jets'n'Guns (2004) from "Rake in grass" developer. I recommend you to watch the gameplay video then try the free demo:
http://jng.rakeingrass.com/screenshots.php