I replaced the two small cabintop hatches with two Lewmar Ocean No. 10 hatches (PN 39610050) from Layline. I replaced the hatch at the forward end of the cockpit under the bridge deck with a Lewmar Ocean No. 1 (PN 39392050) from Layline. The foredeck hatch was replaced with a Lewmar before I bought the boat in 1985. No problems with any of the Lewmar hatches over the years; they're built like bank vault doors.
Henry Mallard at Layline owns a J/30 and can tell you as much as you want to know about various hatches and installation tips.