One way to solve this might be to portray the criminals in question as
foreign imports of some sort.  It would probably be a bit too 
silly to 
make them American, but perhaps they could be Chinese Triad?  If
they've fled Hong Kong after the recent takeover by Chinese government
and are looking for new territory in which to operate, that might 
explain it.  It doesn't have to be a probable scenario, just 
a remotely
plausible one.

That would also give the criminals a strong martial arts angle.  The
Triads are known for their martial artists, since they were originally
a rebellion of Shaolin monk refugees (from what I understand, anyway).
I'm pretty sure the Triad gangs are less scrupulous than the Japanese
Yakuza you describe.  

I hadn't heard that the Chinese had orchestrated any crackdowns on the
Triads in Hong Kong.  Do you have any references for that?  I'd be
interesting on reading up on it.

I don't think you need go that far afield for Triads, though.  From
what I've heard, they're present in just about every overseas Chinatown,
and do especially well in semi-oppressed ones, like Chinatowns in Japan;
basically any place where honest Chinese shopkeepers would be afraid
to contact the police.  What about Triads from Yokohama Chinatown?
Then there's the minor problem of why they're picking on Ukyou...

There are two alternatives I can see:

    1)  If it is important to keep Ukyou as the victim, change the
motivation from a protection racket to Ukyou's inadvertent involvement
in some political or other scandal in which the Yakuza _would_ get
involved:  her father borrowed money from them?  Ukyou's been
campaigning for a non-LDP candidate?  Her father _is_ a candidate?

    2)  Otherwise, try having Cologne out of town, and the Triads
try this on Shampoo.