99% sure this will be a 15mm piston head. Just search KWA piston head 15mm and there are a few results in the UK, plus loads in HK. Lees Presion Engineering also do an o-ring version.
Just don’t put another KWA one on, the same thing will happen. The originals get dry and brittle, and end up like your picture. Get a Cowcow one ideally.