I have the maps for the go, they can be used to activate navigon. Think I got them from sumi, but also gwidion has them. I have navigon installed and working on the 3.0.5 you installed on the Go
Have to check how big the archive is
edit : 1GB 178Mb
There are only the Eu maps, but anyway I think any map/location can be used by tweaking the config files.
Navigon validates having the files by checking the stored name with the stored bytes, so... put 5 dummy maps with the proper names (can be the smallest repeated 5 times), activate navigon to europe and one of those maps, then rename a more recent US or etc map to match the activated one, tweaking the control bytes accordingly.
It should work