On the HarmanKardon site I can find a GPS-810NA_Software_Upgrade.zip. Unzipped it gives a GPS-810NAupgrade.exe file. I opened the file with WinRAR and in the folder NK i see four files.
logo.nb0 logo.sig nk.nb0 and nk.sig.
I hope these are the files you need.
The problem with the GPS810 is that every executable and dll has a corresponding .sig file. Replacing the original igo.exe with another version won't work because there is no matching .sig file.
I haven't found a way to launch another (non .sig) file yet.
My GPS810 is an EU version but I guess that the .sig mechanisms are the same.