No idea on the Airsporter yet, Tone, but what I can give you are the standard Mercury dimensions :-
Cylinder Bore X Stroke = 28mm X 65mm (1.1" X 2-9/16")
Piston weight = 320grams
Transfer Port = 3mm X 11.2mm
Available Spring room when cocked = 110mm
The only difference on my rat gun are the stroke at 70mm and a piston weight of 314 grams.
The spring I removed was a 33 coil Spring with 3.1mm wire.
Another problem which showed up was the fact that the end wall of the cylinder has a hemispherical section machined from the centre of it.
I don't know if this is supposed to be there or is a machining cock up at the factory, whichever it is it creates a little more work as I'll have to make a P38 casting of the cylinder end wall and machine the piston head to match.
That's about as far as I'm going to get today while I stare at the piston and think about the job, but I have a good idea which way this is going to go --- new piston.
All the best Mick