The head shim leak makes them run poorly. It's easy to check, just run some oil under the head and see if it blows bubbles when you turn the prop.

We really aren't having any big issues with the DZ's. Quique has one with over 500 flights and never had it apart. There are several others in the 300 plus range that I know of.

As far as after market gaskets, I don't think there are any for the DZ, and I can't personally recommend any kind of sealer since I've never used any.