The best figures would most likely be the Dragon US 1st Cavalry figures. They represent Soldiers from the 1st Cavalry Division (Airmobile), as your description below shows that your cousin was a member of. This is the figure set:
These figures, with a UH-1D, again available from Dragon (see below), would be a perfect tribute. If you are looking to have unit patches on, both Verlinden and Echelon offer decal sheets that include 1st Cav patches on them.
Dragon UH-1D (review, w/pics here.
Basically any Vietnam era figures would work though. Most figure sets do not represent a specific unit and can be made into just about any unit with the proper shoulder patch. The uniforms, weapons, etc. were basically the same for all units so the figures can be used for any one of them.
Good luck.
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