Puhhh, loads of information here. Which maybe is not very correct or it´s just different when you are from Germany...
So for German Passport Holders as of April 2018:
Visa: either the online thing, ESTA for up to 80 days for the North American Continent (and it doesn’t matter if you go to Canada in between they keep counting down) and do not forget they will ask you to have an onward fly to any destination outside of North America...
OR
You must go and apply for an B1, B2 Visa, fist online tons of questions and then eventually you get invited to an interview at an Embassy, this interview took three Minutes.
If you want to enter the USofA with your own (European registred) bike you must contact the EPA (Environmental Protection Agency,
https://www.epa.gov/ ) and apply for the Exemption Approval (at least we had to for our bikes from 1993 (AT & TA). This is free but crucial to have, otherwise you cannot get it out of customs. BUT this may only apply if you are entering via plane or ship, don’t know… The process takes at the moment 3-4 Weeks, and if you do not have it you must wait at the customs for it (loads of cash to lose for storage, accommodation etc.).
For the insurance, you can have a look here in the HUBB, easy to apply for but takes some time as well…
For the rest have a look around here, all your questions have been answered b4… good luck!
‘Cheers Sascha