Early last year, my 76-year old aunt lent a gas station owner $4000 so
that he could reopen his pumps. The owner, my aunt, my mother, and
myself signed the promissory note, my mother and myself as witnesses.
The agreement was that he would pay back the $4000 in full, plus $200
flat interest within one year. The note was not notarized.

He has not made good on the contract, and still owes approximately
$1500. This situation has caused considerable stress on my aunt and
mother, as well as their relationship. The gas station owner has been
giving them a runaround for months, promising to have the money, yet
never does. I have doubts that he could afford to pay if legally
forced to.

I'd like to seek legal action against him, but neither my mother nor
aunt can afford it. How much could I expect it to cost? What are the
chances of success? Is the promissory note enforceable in court, and
even if they find in our favor, what is the likelihood that we'd
actually get the balance owed? He still owns his gas station/auto
shop, so I doubt that he's bankrupt. Is this even worth pursuing?

The note is still enforceable, between the parties involved, despite
it not being notarized.

Likelihood of recovery?  Hard to gauge.  

Since you still have the promissory note, the burden is on him to
prove that he has paid the amount.  Sucks if he holds the note :)

Take him to small claims court, and get a judgment.  After you get the
judgment, any local sheriff/police or attorney can tell you how to
collect by attaching his bank account.

4KEITH (I'm NOT a GOOGLE Researcher)

You try to use a lawyer as a mediator first.  It's a whole lot cheaper.  

Another good option is a small claims court.  Since it is a business
loan, you may be able to sue the business entity if more than one
person owns the gas station now.

It is enforceable.  However, if you wait (depending on your state) 2-4
years after the note was overdue (after the year expired) you may lose
the opportunity to sue.  All you need to do is go to small claims
court and have the matter reduced to a judgement.  That in hand, you
can pursue collection by any number of means, including lien against
bank accounts and the assets of the station.