Here is a difference between the GXH50 sold in US and that sold in UK first is the QED4 version bought in the UK and the second is QXA sold in US I am not too sure if this is sold in UK but it is something to look put for.
I asked HoughMade "Did you find that the hole on the grubee 2 gear box line up, I have a GXH50 and a Grubee 2 gear box but it is badly out of line with the holes."
My problem was due to the mounting holes to allow the engine to be bolted to equipment like the cement mixer etc. These prevent a snug fit and keep the holes of the Gear box from lining up.
Just some info I hope may be useful to others, for what it is worth.
Ed