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The 63mm Secondary that I mentioned previously is the correct size for your primary mirror . Whether the mirror is an Skywatcher of 1200mm f/l or the GSO 1250mm f/l shouldn't matter, the 63mm secondary suits both, unless your tube is oversized for some strange reason.
If you could post some photos of the tube and mirrors and their coating condition it would help. Removing the old secondary from the holder can be a problem as they are usually held on by a silicon glue which is tenacious stuff.
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