Posted: A kind note to the community who chose to edit my spellings of the word nonstandard to non-standard... Nonstandard is the way I spell this word. More importantly, it is spelled this way in Collins, American Heritage, and the Unabridged M-W. The form non-standard is not found in any of these three American dictionaries. In other words: If it ain't broke, don't "fix" it!
The answer proper:
'Broken' is the past participle for 'to break'. It is being used as an adjective in
If it isn't broken, don't fix it.
'Broke' is a nonstandard past participle for 'to break'. Reference. That is, it is considered to be nonstandard from the point of view of 'standard English'.
To avoid confusion, let's just call 'broke' colloquial for 'broken'.
Since ain't can also be considered colloquial, it makes perfect sense to find them together in the sentence
If it ain't broke, don't fix it.
Thus, ain't does mean isn't here.
As an example usage, in the "old days" of the cathode ray TV and before cable or satellite TV service, one had to adjust the rabbit ears antenna to get good reception on the TV (pronounced TV in this example).
Caution: nonstandard/regional language ahead:
Now suppose them rabbit ears broke? Well you're gonna use an aluminium fishing poll and duck tape to rig a new antenna. Now the TV works good again. Your wife don't like it, and she wants to buy new antennas. But since the television is working fine with the antenna you made, you say "Martha Mae, if it ain't broke, don't be trying to fix it. We can use the money on catfish bait."
However, one dictionary calls 'broke' an archaic past participle of 'to break'. If that is the case, then at one time it was not considered nonstandard. I have to check the OED for further details on this.