Four addtional textures (Navy, Army Mule, RN & RCN) that fit the HUP AI perfectly can be found in the FS2000 HUP package by Rockster on the following page: http://www.simviation.com/fs2000heli16.htm
BUT you must keep the current Prop2.BMP in the texture folder.  Otherwise all you need to do to change textures is to copy the files you wish to use into the HUP AI texture folder.