uh you might want to check that file a little closer, I tried running the game with it and it gave me this error
SCRIPT ERROR: GDScript::reload: Parse Error: Parse Error: Unknown character
At: res://files/scripts/mainmenu.gd:16
ERROR: GDScript::reload: Method/Function Failed, returning: ERR_PARSE_ERROR
At: modules\gdscript\gd_script.cpp:507
I had this same experience, if I extract the game it functions before overwriting the mainmenu.gd with the modded mianmenu.gd and afterwards it cannot proceed past the warning screen.
If you press Confirm you get ERROR: Object::emit_signal: Error calling method from signal 'pressed': 'Control
(mainmenu.gd)::_onwarning_confirm_pressed': Method not found.
There may be confusion involved in what version would be compatible with the mod? If I open up the original vs the modded file there are actually a lot of differences in areas I wouldn't have expected.
Here is an example of one such unexpected change.
Left is modded, right is original.
This difference of set.hidden(false) vs .visible = true (and vise versa) seems pervasive in the document, among other similar syntax type changes.
(I believe) I was able to achieve the goal of the mod by doing find and replaces of:
"player.beautybase = 40" -> "player.beautybase = 100"
"startslave.beautybase = 40" -> "startslave.beautybase = 100"
"Foster " -> ""
(ignore quotes)
Highlighting differences in the files via your mod showed me exactly where to look for these variables, so I genuinely thank you for sharing your file and the idea even if in my case it didn't function as intended "out of the box".