It's a 31.8 - Pretty standard, more likely to be too big than too small, in which case a shim will fix it.That would be amazing mate. I don't know what seat tube size is, what's the chances the front mech will be the right size?
Sean
If it is too small you just need to source an alternative, which should be fairly easy.
If that 200GS FD is the original check to see if the diameter is marked.
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