Welcome to the forums 1gop1
If an original MGC spring proves difficult to find these days and you've still got the original spring you might be able to match it up to a suitable replacement by counting number of coils per inch, and the wire thickness.
Both of these together with o/d and an approx free length can help find springs from on-line suppliers or your local hardware store
Good luck
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Cerwyn
Hobby collector of Replica model guns and Militaria.
also member of Living History Reenactment Groups.