The smaller 1500cc engine is from the Corsa GP Turbo.
http://www.cardomain.com/ride/2081027 Its actually a 3E-T the cousin of the 2E-T from the EP71. They convert the 12v head to that of the 16V FE head. Its a simpler, stronger conversion than the normal 5E because the 3E is already turbocharged from factory.
Here's the source of that Jam vs Boss video.
As for the Teins, people have done it here. They simply buy different spring rates and use as per surface. For dirt my friend uses 5/4 and for circuit he uses 12/10. They work fine.