Crossfire was originally on the Apple II and later ported to various other systems like C64, Vic-20 and PC so a search might reap results for other systems.
Thanks to Atari's easier to decypher bitmap layout vs most others it would probably serve as a good starting point, though it does use Graphics 7.
I never knew but there's also a monochrome version, possibly a more direct port from the A2 original.
The game fits easily into 16K which makes the job somewhat easier too.
The gold standard for reverse-engineering and getting workable source code back is IDA Pro but there's various freeware disassemblers that could also do the job. Whatever way you go there's going to be work for you like properly deliniating code/data sections and commenting and putting more meaningful labels in.
It looks like the C= Plus/4 doesn't have this game so it might be a good candidate system for a port. But it's graphics mapping is much the same as the C64, so the C64 version would likely be a better candidate to port over.