The guy that did the G10 swap is "fujioko". He's on here but hasn't been here in a long time. Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - View Profile: fujioko
He did a B3 swap from a Festiva on another Miata. The B3 is a de-bored version of the B6 in the Miata, SOHC 8 valve too. All the B series engines share a bell housing bolt pattern.
Festiva guys basically give away B3's.
Looks like he also swapped an RX7 5th gear in. Not sure how much difference.