> It is my understanding that the water-boxers from '83.5 --> '87 had some
> design flaws resulting in head leak problems, but the '88 --> '91
> water-boxers had this problem fixed.
> 
You information is flawed.  The engine did change for the 1986 model year,
going from a 1.9l to a 2.1l engine.  Some cooling system changes were also
made.  But 1986 and on are basically identical.  *All* of these engines have
head troubles.  It is claimed by many that this is a serious design flaw,
from the rather unconventional head seals.  Recently somebody characterized
the engine as a "student" attempt at making a water-cooled engine out of
an old TypeI VW engine.  It is close to the truth in my opinion. 

> Questions:
> 
> Will an engine from an '88 --> '91 Westy bolt right into the '83 --> '87
> Westys?
>
Well, yes, it will.  But there were two other changes, the change from Digijet
to Digifant fuel injection and many cooling system changes.  The engine is
slightly larger, but not worth changing.
 
> And if so, what will one pay for a good rebuilt '88 --> '91 Westy engine?
> 
I'd suspect all Waterboxers cost the same rebuilt, but you need a core engine
to avoid core charges.  Typically, you can't give a 1.9 engine in exchange.

> Are new '88 --> '91 engines available?  How much?
No idea.

There might be a few changes made in each year, but not anything significant.
Interestingly, the 1988-1991 engines are rated 5hp *less* than the 86-87
engines.  This appears to be a change in how the power was rated, though,
although it is possible the FI "brain" was de-tuned.  Almost all 1986-7
busses have the later brain -- it was the subject of a recall, and replaced
with the one from the later busses.

If you have an 86 or 87, take heart, they are no worse in the engine department
than the later ones.  If you have an 84-85, don't worry, either -- the later 
engine is a bit larger, marginally more powerful, and fits.  But it is probably
not worth the bother.  They both have head problems.

All this said, I have personally never had *any* head troubles on any of my
water-cooled Vanagons, 3 1986 versions.  Two of this have well over 100k miles.
I've been lucky I guess.

malcolm