For twisty roads in Japan (or anywhere else), you won't do better than the Saturno model 500 or 350cc.
I've got 2 and they are the best handling bike I've ever ridden and I'm coming up for 62 years of age shortly.
Our Gilera Owners Club (UK) does have a list of known problems for the Saturno (nothing too drastic, but better to be aware of possible problems, so they can be checked/fixed before being major disasters). If you can email me email@example.com
I will email you a copy (it is in pdf format). Our Club also has workshop manual in English available (re-typed and pictures digitally enhanced) for sale from our web site.
Our Club is also trying to make a complete list of every Saturno built 1988-1992 and as you are in Japan, may I ask for your assistance? I have tried numerous times to get a post in one (or all) of the various Saturno forums in Japan, but have had no success, so would it be possible for you to post the following (in Japanese if possible) on as many Saturno Forums as you can find in Japan;
The Gilera Owners Club of Britain is trying to make a list of all Saturno built 1988-1992 and ask if you could assist us by sending the following details of your Saturno by email to firstname.lastname@example.org
1. Year built
3. Engine size
4. Engine number
5. Frame number
6. Town/City/Country where Saturno is
7. Used for road or race
8. Your name if you wish it to be shown on the completed list
Thank you for your help
If you are able to help us, it would be very much appreciated as the vast majority of Saturnos built were for the Japanese market and many are still there.
I Gotta Gilera Owners Club of Britain