The FASTA format is very loosely defined - all it needs is a single line header with a ">" and sequence in the next line. You can join/concatenate all fields except sequence using a delimiter (say _ or __ or even blank space but I recommend using a non-blank-space delimiter). Append this concatenated string to > and print it, then print sequence in the next line. Rinse and repeat.

I've outlined the approach for you, writing the code should be trivial.