Exciting! Can't wait for the great changes of the already great tool.
Just a suggestion though: maybe move it to SDL2 and fix fonts first and then start the redesign and all that? Here's why: some users like exactly what they have at the moment and that's what they've paid for. Changing the core tool's elements design will change everything for the end user.
Having the current version with fixed major issues (like the fonts) and stopping its support along with an announcement after would probably be a smoother move from the customers' perspective, keeping the built trust. Then just do whatever was initially decided already and everyone's happy in the end.
That being said; I will support you and whatever you do with this tool. Good luck with the new code and I hope you will have a lot of fun with the new riddles :)
Kind regards.