The spring is the same as used on the Airsporter. The spec varies a little from 'authority' to 'authority' but for a standard spec round wire spring, which you almost certainly have:
21mm od x 14.5 id x 3.25 wire diameter approx 265mm long with 29 coils. If you go shopping for a new spring you may find Titan brand ones advertised for the Mercury. Note that these will no doubt fit the piston but will certainly be too long and you will not be able to fit it unless you are exceedingly strong. It will need cutting down and the end refinishing. Protek Supplies are currently advertising Airsporter / Mercury springs I believe. Or Chambers or John Knibbs.
The stuck piston you mention is a well known issue. The cause is almost certainly the buffer washer disintegrating and jamming the piston in the cylinder. You may be lucky and be able to pull the piston out or carefully use a drift to tap it free.
Good luck, Phil