One online tool for BASE64 is Motobit Software's BASE64 encoder/decoder.
To use it to convert StarTools masks back into importable PNG files;
This should result in a download of the mask as a PNG file which can be imported into the StarTools mask editor, as welll as other applications.
Even the way StarTools displays and scales images, has been created specifically for astrophotography.
There are a great number of tools and techniques that can be applied in StarTools that let you home in on a good colour balance.
StarTools enables you to keep M42's colour constant throughout, even in its bright core.
You can convert everything you see to a format you find convenient. Give it a try!