The second, 'broken' is grammatically correct. However the first works too in more informal language. A common colloquial form of this expression is "if it ain't broke, don't fix it". Hardly correct English! But perfectly acceptable as a colloquialism.

"Don't fix it if it's not broken", although the first one is commonly heard.

Don't fix it if it's not broken is correct. The first sentence is bad English, even though one might hear it from some people.

It should be the second one. “ broke “ is the past tense form of “break”. “broken “ is the past participle form of “break “. In passive voice, the past participle form is used after “be”. 😁