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.pxt and Words.pxt.
Editing map tiles is done by editing map1.pbm:
Each coloured pixel represents a 16x16 tile using
the following colour coding: