On the evidence available in this picture I could not declare it to be a GX engine, ... for the following reasons.
The rocker cover does not have the second fuel line support fixture near the rear. This is to support the fuel line for the second Hitachi carb.
Not GX rocker cover.
Cylinder head does not clearly show the four large coolant holes under the inlet ports. The later A14/15 heads have three small ones & a clear close up of this area may reveal more detail.
Cylinder head is in question.
The short block assembly differed from the stock 1200 by virtue of its flat top pistons in the majority of engines, the camshaft & the additional fitting on the inlet pipe for the water pump. I can't see any obvious sign of its removal so again, for me,
I can not identify the block as being an original 1200 GX.
I am not saying that it isn't, but the prime identification features that I look for in the picture are not eveident.
