Assuming you've got the larger GP Skybolt, I'd vote for the cheaper but still good quality/power Magnum 91 4c or the Surpass 91. I flew my older and heavier Sig Skybolt with the now extinct Magnum 80 4c. It had plenty of power for scale flight at half or 3/4 throttle. Most biplanes don't hover or harrier that well to warrant overkill on engine size.
Main rule of RC -- only 4 strokes in Cubs or Bipes and fly at scale speed.