I did a little research and found out some recent info. Keep in mind that I have not personally used these sources, but the info should be reliable. There are still no major car rental agencies in Katakolon, but there are some local companies. You can try to contact Sakis Diplas or his son Chris (they apparently own a local agency) at diplas@katakolo/gr . If that e-mail does not work than you can try calling from the USA by dialing 011 30 693 245 2648.
Another source suggest you call Dionysius at rentalcarkatakolon.gr by dialing 011 30 2621 041 727. I am told this agency is opposite the dock and only costs about $75 per day. I would try them first.
Another option is to arrange for a taxi in advance. Some have had good luck with email@example.com
George should quote you a price to pick you up at the port and take you to Olympia and back (or anywhere else).
I have personally driven in Greece and on the Pelopnese and never had any major problems. But keep in mind that you want to have a decent map or very good directions. The biggest problem is that in some areas all the road signs are written in the Greek alphabet....while some other areas also have English alphabet signs.
Best of luck. Its sure worth a few dollars to make the phone calls.