I have a misfire in one of the cylinders that is resulting in
incomplete combustion and higher than normal emissions.  I have had
this problem for about 2 years and passed CA emissions last year with
the help of a brand new Cat.  I want to solve this problem within the
next year before I have to take it to emissions testing again.

Cylinders 1,2,4 show 150psi on a compression test
Cylinder 3 is 130psi dry.  did not try wet

About 3 months ago, I found a jammed lifter on cylinder 3 and replaced
it with a new lifter.  I believe my valves are set properly.  I set
them when the engine was warm.

I am getting a used long block in the future and am thinking of
dropping it into the engine bay temporarily so that I can inspect my
current engine at a more leisurely pace.  If I take out the engine and
take off the heads, is there a chance I will be able to see anything?
I am a novice and I don't know quite what to look for.  I have a
almost-new reman spare head and engine gasket set in my garage that I
can swap onto original long block.

What I've changed so far:
  - all new Bosch spark plug wires
  - all new Bosch platinum 4+ spark plugs
  - new rotor and cap on distributor
  - switch 4 fuel injectors with four different injectors known to be working

I have taken the vehicle to two different mechanics who have not been
able to identify the source of the problem.  One of the mechanics was
Fred's Garage in Redwood City, a place that specializes in vanagons.

I'm thinking that I can swap the head on, but I'm not sure if it will
solve the problem.

Engine power is fine.

Thanks for any advice.