Excalibur crossbows

[Talon]

The Meteor really isn't a full power rifle Pete.It can get near with work but you'd be pushing the rifle and it wouldn't be pleasant . I'd avoid Titan springs . They're always too long and it will need cutting down . Older springs were much softer sprung , the Titans made stiffer and will be stiffer still when cut.And putting a larger Spring in the meteor will most likely result in a loss of power ,that's if it will even cock . It'll also put undue stress on the cocking mechanism and the trigger .
You'd be best just stripping , degreasing and relubing. You could also try and source an original spring and piston seal . That might get you up near 10.
My advice would be first get a chrono and check what the rifles putting out . It might surprise you, If you want something that'll comfortably do 12 then I'd suggest something a little more efficient like a hw99 or 95 , There's plenty other makes that'll be fine ,but for quality I'd go with Weihrauch.
I wouldn't personally touch a modern BSA Turkish Spring rifle .If you wanted to stay with BSA there's lots of Birmingham made Supersports still out there for not a lot of money .