English does have the loanword https://en.wiktionary.org/wiki/id%C3%A9e_fixe#English but I think this is often considered technical vocabulary (per Wiktionary, in psychology and music composition) that most native speakers would probably not recognize.

I meant that idea and fixed are both English words, so it should be possible to associate ideafix with "fixed idea" - a bit more obscure though than the original.