Which editor for MIDI?

What kind of editor is needed to write a MIDI file of
<http://www.rev.net/~aloe/music/19807221.html>? Most MIDI programs won't
even accept the key signatures at
<http://www.rev.net/~aloe/music/19808192.html> and
<http://www.rev.net/~aloe/music/19809102.html>. It seems that a good
hexadecimal editor ought to suffice, if I can find one. The one I have
insists on changing '00' to something else, and won't accept interior
inserted bytes.

What can MIDI do, and what can't it? For example, if the purpose is to
create random output to estimate the percentage of possible music that has
already been written, is another code preferable?

-- Beco dos Gatinhos <http://www.rev.net/~aloe/music>