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Old bridges and rivets (again)

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I have been asked to rate a bridge built in the 20's or 30's. It is a 66'
span single lane bridge with 5' deep trusses on each side. The trusses have
built up plates for top and bottom chords rivetted to double angles with
rivetted gussets plates and angle webs. Typical stuff, pony type truss. A
few of the rivets have disappeared in the top and bottom chords. Some of
the double angle webs are stitched together with rivets and the corrosion
between the angles has pushed the legs apart between the rivets. 

My question is how would one assess the existing rivets of which there are
2 every four inches in the top flange and similar in the bottom. If some
are missing this suggests their condition is not great. Are there any good
reference guides anybody knows about for this kind of thing. 

Would it be common to do some welding with this kind of truss or is there a
concern with chemistry? I am concerned with the capacity of the existing
rivets but don't really know how to assess. If they only want to rate it
for a few tons then I am not worried. However, they may want to get all
they can out of the structure. 

The pitting in the top chord is severe. The edges are curled due to the
corrosion between the plates. Little or no maintenance has been done on
this thing for 20 years. The deck beams are completely rusted through at
almost all locations. These are to be replaced. Deck is wood and in not bad
shape. 

Any thoughts as to rating methods for these main truss methods other than
an estimate of present thickness. I am pretty confident about the steel
grade. Also, does anybody have a good reference for the top chord strength
of a pony truss with the lateral support being provided only by the
vertical struts from the deck beams below.
i.e.

     ---                ---   
      I                  I
      I                  I
     II                  II
    I I==================I I
    I I==================I I
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David Handy, P.Eng
Ontario, Canada