Ikachan In-Game Editor
How to access it, is to hold S while choosing NewGame. Or load your Saved Game position by doing the same, but while choosing LoadGame. This editor only edits objects though.
How to control:
Mouse or Arrow Keys - Move Cursor
Left Click - Create Object / Change Box Flag Up
Right Click - Delete Object / Change Box Flag Down
Z/S - Save
More information is available here
For other changes:
Editing Credits and the Beginning New Game Instructions is possible by editing Staff.ptx and Words.ptx. Events can be changed by editing Event.ptx.
Editing map tiles is done by editing map1.pbm (a renamed .bmp file):
Each coloured pixel represents a 16x16 tile using the following colour coding:
Not all image editors will save the file in a way that is readable by the game though, so be careful.