anything 25 years or older will go right past customs as long as you pay the import duty which is a reasonable 3% of value on cars and 25% on trucks. I've never been able to find the tarrif on mopeds, if one even exists.
Anything newer then 25 years, save your money. You can get it here, but having it approved for road use is like pulling teeth. Unless it's on the list of cars someone's already had a registered importer go through the process on, which is very very small. it'll set you back an easy 10k, That's for cars of course, mopeds they may not even check.
Now of course, if you live in a state that doesn't require registration or titles, you can just import it for parts. When you do this with cars/trucks they usually make you cut the frame so it's unusable, but again .. mopeds ... probably not an issue.
I've pulled 3 VW's europe, and i can tell you the one that was 24 years old ended up being so much trouble i ended up letting it sit in belgium until it was officially 25 years old.