I think the A100 is almost certainly this new 2.0T - Toyota is very proud of that engine so far.
As much as we may all lust over Mazda having an I6, let's also be honest - it sounds good on paper but that engine seems pretty pedestrian and hasn't been anything close to a B58. Sure Toyota could use the block and do their own internals, intakes, cooling, etc. but I just don't see that happening when they seem very excited about this new 2.0T and in their own words, called it the successor to the 2JZ -- which the Supra made famous in the first place.