How much more power?  I can suggest that the heads that on fixed on the
waterboxer, ie new AMC heads or just JB weld and or Ceramic Coated heads
really seem to be the final cure for the head leak.

As for Power, I'm too in search of  power. So here's my short list

S&S headers-- 4 cylinder engines live only if they scavenge.  See their
site and Header 101 Tech article

Better flowing heads, Boston Bob's Big Valve Stage III

There are others, but for now consider those as things you can purchase
and assemble. There are still some consderations so get back to me if
you like.  You can read some of my other suggestions under Kelphoto as
author search on vanagon.com archivies. go back to 98-2002, haven't
written a lot lately, doing a home addition etc etc.

Mark Keller
91 Carat