Yeah it's easier to make your own levels than load the old ones at the moment, and that's by design. Loading of the original levels is not out of the question but would require a tonne of work, re-interpreting the binary format of those files. In fact I don't even think the binary format of the GOB files is documented anywhere, so it might not even be feasible...
To convert old levels you can work it out from the existing files - the foreground is no longer a simple black & white mask and is an entirely separate PNG image that's just laid over the top. So you can open the old mask, "select colour" and select all the white, then use that selection mask to copy from the original background and paste it into a new file.
Godot can't play MOD files! Believe me I tried a lot of approaches, but in the end I'd have to build MOD support into Godot myself. Which of course is another huge unjustifiable endeavor. It broke my heart to convert them to OGG files but that's what's it's playing right now. If I make a new version in another system it'll be MOD files all the way. Still Godot makes it far more feasible to make 2D retro games in a more timely manner and I highly recommend it.
Yep I can make the other things customizable (OGG files for music, WAV for sound, horizontal 16x16 PNG strips for sprites) but I was going to gauge interest before putting the extra work in. It's not complex but it's not free in terms of how much time it would take to make it load externally and get it all re-tested.
All that would definitely be in a version 2 "paid update" but yeah I was going to see how this goes.