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.
As of 1.7, enhanced mouse controls are implemented; Zoom in.
You can convert everything you see to a format you find convenient. Give it a try!