I'd thought of that when I first pulled it out for the first pictures. But since most of the moving parts in a standard gearbox aren't present in this one, it's kind of useless even for that.
Here's some pics of the parts I'd hoped to replace, but can't
Standard piston on top, Firepower piston below.
Standard cylinder, cylinder head & nozzle on top, Firepowers All-In-One version below, with attached "Tappet plate"
End on view of cylinders, standard on left, Firepower on right
Piston heads, standard on left, Firepower on right