Wow, You are fast with Your answers, faster than I am (re)connected to the internet !

As I want to keep to be legal, I will try the contact Gail Goodman of the Motorcycle Express insurance.
Meanwhile I got an answer of the insurance 'THE SUNRISE GROUP'
>>
[...] falls Sie auf einem Visa waiver nach USA einreisen koennen wir bis zu 3 Monate versichern.
Um eine Police auszustellen benoetigen wir, von allen Fahrern :: Kopie des Reisepasses, Visa, deutscher Fuehrerschein & Kopie Kfz-Brief oder Schein. [...]
<<
(www.sunriseworldwide.com)
They asked me for the 'Kfz-Brief' (means the german title) OR the 'Schein' (means the german registration document).
I can use the title, which is a valid document, but without having the registration. Therefore the question is, if this insurance would pay in the case of an accident ?
Without asking them exactly I might have only the policing paper but no insurance at all.

For my other idea of import/registrate:
> It would be easier to answer this question if I know what kind of bike it is

It is a Yamaha XT 600 2NF, which is the low powered version to the XT 600 2KF.
For the model year 1988 I have not found my XT version on the Yamaha web sites (http://www.yamaha-motor.com/sport/parts/home.aspx), only the model names XT 600 U and XT 600 UC.
This should be the same models, but they have different names. The parts lists have another sign and are equal to the german ones of my model.
Because of my connections to persons living in Texas I would try to import and registrate my XT in Texas.

> Also, how many digits are in the VIN number?

The vehicle ident number has this "2NF" followed by six digits (and a separated seventh digit, which is not marked on the frame).

Is this XT a motorcycle, which was sold in the states and which has the 'US DOT and EPA sticker' ?

Thank You until now, hoping for some more details, Gerald