@mima >Someone is also trying to do a free reimplementation nof EoSD's engine in Python That could be free software if EoSD's engine doesn't have scripting support, or the scripts have been replaced.
According to Arch AUR the license is "GPL3", but as the source repo is down, I can't check if that's GPLv3-only or GPLv3-or-later.
>fully reverse-engineered source is libre enough to you Decompiled source code of proprietary software is not free software, as that's a derivative work of the binary and that binary is under a proprietary license.
Also, the result of such process is usually illegal to distribute - although there's little chance of ZUN suing in this case.
The README of the de-compiled sources repo say the license is "GPL2", but it's unclear if that's GPLv2-only or GPLv2-or-later and that is clearly not the license of most files ("hex rays" has even claimed copyright of de-assembler output, under no license, for example; https://github.com/nmlgc/ReC98/raw/master/th04_zuninit.asm).
Someone is also trying to do a free reimplementation nof EoSD's engine in Python at https://pytouhou.linkmauve.fr/, but it's very incomplete (like you can't use bombs for example).
And if fully reverse-engineered source is libre enough to you, you can also play HRtP in a free PC-98 emulator like NP2kai and DOSBox-X