You have at least a couple options. 1) you can mill the 27.0mm race seat to 26.4mm to fit an ISO spec headset, or 2) you can fit a JIS spec crown race with ISO spec cups. Many Japanese headsets are available in both ISO and JIS spec, and crown races are sometimes available separately. The head tube fittings are the same for ISO and French headsets (30.2mm), but JIS head tube fittings are slightly smaller (30.0mm). If your LBS has the proper tools, getting the crown race milled to ISO spec is probably the simplest option, as you wouldn't have to dig around for a separate crown race. ISO spec also makes a wider variety of headset choices available.