Guidelines for forking
A fork of the source code is when you take a complete copy of the source code and develop it separately from the original version. You are free to fork the OHRRPGCE's code any way you want, as long as you comply with the GPL license. There are already several fork projects in the works, such as big porting projects like Jormungand, which indend to bring the OHR to new platforms, or simpler forks, like a special version of GAME.EXE with no battle system and extra-large plotscripting buffers.
If you decide to create your own fork of the OHRRPGCE, it would be courteous (but not required) for you to notify James of your plans.
Also, it is good to try to maintain compatibility with other versions, so that end-users won't be put in the difficult position of having a game file that only works on some version of the OHR but not on others, without any way of knowing what is the correct version.