maybe the rebel 450 motor would be a good fit =)

*edit*
Scratch that it would be way too much work =(
The best bet might be to get that 400cc motor imported.

But if your going to be spending all this cash on this bike you might as well just get a used honda shadow vlx and bob it.
I find the vlx all over the place with less then 900 miles on them for $2000-3500, and for just a few more dollars you can convert it into a bobber.
Or even a honda rebel, they are very easy to bob and are less then $2k