Definitely looks like you bought a dodgy subframe there. Somebody would have cut it for no apparent reason. But on the other hand, the later Corse fitting did not involve the carbon bracket you are trying to use and the ECU was living more towards the front (on real corse bikes) or by the battery (with the 5.9) for the 998 series. That probably explain why this was chopped off. See my earlier posts on the same thread and you will see a standard SPS subframe on my 996 and 998. The 998 has the carbon seat support fitted as well.
That said, even if you fix this, I think that you have the wrong end of the stick. If you are to use the 1.6 ECU, you are much better going with a 748 R subframe. This is designed to fit the 1.6 ECU (and it is Alloy).
Please see attached picture of a current build I am doing. If you observe the rear you will see that the design is different to accomodate the 1.6.
Incidentally, as well there is a carbon seat support design that accommodates such subframe. It has some curly brackets at the front going back to bolt on the subframe. I don't have a photo of such one as I (stupidely) sold mine some time back. But if you do some research you may come across one.