I thought the center bore on our cars was 60.1 if it matters to anyone.
Anyway, I've been told, but never confirmed, that the Lexus IS-F spare works on the MKIV Supra for this purpose. I'm ashamed to say that, even though I have one, I've never taken a look at the spare. I'll do that the next time I go to storage because, if it can work on the MKIV, it may work on the MKIII and accommodate BBKs as well (OEM 6/4 piston set-up on 19-inch factory wheels.
Not a lot of these cars were sold, but it's worth it to see if we have another source for space saver spares. The Lexus SC 300/400 twins could be another potential source as well (assuming they have space saver spares).
Ken.