Don’t bother with a collection agency, they’ll only eat up a big percentage of what the deadbeat owes to you. I’d suggest going to a magistrate and filing a claim for the amount of money owed to you for the work done to this point plus storage fees if (and only if) you had the terms of your storage fees printed on the repair order that the customer signed.
If you don’t have repair orders or a storage fee that is posted then you should contact an attorney for advice. You could put a mechanics lien against the car. From what I understand this gives the owner a certain period of time to either pay off what he owes or the title will be transfered to you and you can dispose of the car any way that you see fit.
I hope you have something in writing documenting the original work, the estimated cost and your policy on storage fees. You should also get all the paperwork in order that shows how you’ve been trying to collect money from your customer over the past few months. The more documentation that you have the better your case will go.