in addition I would need two variations of the english "where there's a will, there's a way": "where there's a way, there's a goal" and "where there's a goal, there's a will". Could these variations work with "fonn a níos fiach"?
Fonn a níos fiach just means where there's a will there's a way, nothing else. The English sentences you mention aren't proverbs so there is no existing corresponding Irish phrase - you would have to make something up, and it wouldn't sound like a proverb then.
even if it doesn't sound like a proverb, is it possible to change only one or two words (as I did in english) to obtain the different meaning?
like: desire makes you go hunting. Hunting makes you know, what you hunt for. Knowing what you hunt for, makes you desire.
isn't exactly the same. in another forum i got these translations, which i really like a lot:
Áit a bhfuil toil tá bealach. = Where there’s a will there’s a way. Áit a bhfuil bealach tá aidhm. = Where there’s a way there’s a goal. Áit a bhfuil aidhm tá toil. =Where there’s a goal there’s a will.