If i can get the project to open in GMS i can take a look and maybe refactor a bit (are u using scripts, macros, globals, state machines/enums, parenting?). Could also load most of the variables and such from a text file (.ini) so its easy to mod without even opening gamemaker. I had a very similar thing happen on still not dead, i smashed out the prototype in a month and after that had a hard time even looking at the project it was such a mess -__-' lessons learnt, i think in future i'll try to clean the code as i go :P
i read this: https://steamcommunity.com/app/214850/discussions/0/1326718197209180846/
Seems i cant open a GMS2 project ;__; maybe theres some kinda work around..