I suspect that the crank seizing at one point have irreversably damaged the bearing races on th BB cups to cause the clunking feel and sound you are experiencing and the LBS cannot find an exact replacement to fit the bike as it might be a proprietary design. I'm sure a resourceful mechanic can figure out some way to adapt a more conventional BB to the bike, but it might cost more to do than the bike is worth. Proprietary designs are always a risk because of this. Its similar to the whole Betmax/JVC thing when VCRs were in it's infancey. Some design just don't end up as a standard and eventaully just fades away into oblivion. Your bike is "Betamaxing" on you....

Chombi