If this is just a 1-time thing you need to do with this image that doesn't require a reproducible optimization, you could use drawcircle to manually "fit" the circle to your best estimate.
circData =
Circle with properties:
Center: [490.5 441.5]
Radius: 204.38
Label: ''
Color: [0 0.447 0.741]
Parent: [1×1 Axes]
Visible: 'on'
Selected: 1
