Is it the Pumptec 205 head?
If so, I think Ashbys is around £39 + plus Vat and del for the piston and seal kit.
This will fit any 205 head regardless of the psi. I think the psi depends on the motor that drives the pump
and also the control valve....perhaps.
If running in a Ninja, it will be 400 psi max or the airflex would be 600 psi max.
I'v found over the years that just the piston and seal kit needs replacing. I have never found the valves need replacing
so usually just the piston and seal kit will suffice.
Probably also advisable to replace the offset drive bearing @£19 if machine has done 6 to 12 months work.