Motorworx took a 1600cc B6T from the Mazda Familia 4WD rally car to use as a base. The bores have been enlarged 4.5mm upping the displacement to 1732cc. Pistons are Arias forgings for the Toyota 4A-GZ.
We are going to take a further look at this on miataturbo.net.
Comparing some ebay rods to bp rods to b16 (assuming i can find some floating around with my honda buddies).
Check this thread for how it pans out... http://www.miataturbo.net/forums/showthread.php?t=8404