If you fit a new Speedo head it will need to go to the Dealers ( or someone with programming equipment ) As the immobiliser is usually in the speedo head. ( unless the jap version is different which I Doubt ) I would think the Immo is the same in both countries unless its turned off in Japan. ??

I would fit a face or speedo converter. As I would think that the Dealers would tell you that they cannot Tell you which MPH Type Dash fits to that car as its listed to have the KMH dash fitted. ( You know that dealers can only supply parts by part no, they have no idea if a number isn't listed )

Personally I would think that the job isn't worth getting involved with doing part of the job. ??
Like everything else it will take you longer to find out how to do it and what needs doing than to actually do the job.

i wonder if the device to alter Anti theft to uk spec is the frequency of the remote controls. ?? as uk is usualy 415 MHZ and Foreign are 315 MHZ

Is it Really worth the Hassle. ??