Is it the connector or the header on th power board? Have you checked the back of the header pins to see if any of the solder joints securing the pins look broken, cracked or dull? If any of them look suspicious, just reflow the solder by melting that joint then put a tiny bit of fresh solder into it. Probably wouldn't hurt to reflow them all on this nearly 30 year old power board anyways....

If that all looks okay and you're sure it's the actual IDC connector, you can buy IDC sockets from Pinball Life and I believe GPE, probably the other usual suspects like Marco as well. They also sell the IDC punchdown tool which allows you to push the wire evenly into the blade of the socket to make a good connection.