I am pretty sure most Garmin GPS's will work just fine. They do sell worldwide maps via CD or microsd card for these units..

I would look at the Dakota 20 - small enough to use as a bike gps but larger in size vs bike specific gps's.. I own the larger version which is the Oregon series and while it works great, is a little big for a bike computer.. The biggest problem with many of the Japanese specific maps is that they will be in Kanji. You can get a worldwide map set but I doubt it will have the specifics you may desire.. There are many GPS websites to throw questions out to: http://gpsunderground.com

reviews:

http://www.gpsinformation.org/penrod/dakota/dakota.html
http://www.gpsfix.net/gamin-dakota-2...t-impressions/