Assuming you are considering an F-105D, the most produced version, scale length at 1/6th would be almost !30". I doubt a 140N/32 lb thrust engine would come anywhere near that needed to power that large and heavy an airframe through a normal flight regime. I'd speculate the 220N/50 lbs would be a reasonable starting point. Also, 18 KG/39.6 lbs is likely an underestimate for that size airframe. My estimate with any external stores and an adequate fuel load for the large engine will make the gross weight in excess of 25 kg/55 lbs. That's why I mentioned 1/7 th scale. Just my thoughts. A THUD 1/6 th size however, would be an awesome sight! The F-105F version (2 seats), used for aircrew training and Wild Wiesel, I think, was even several feet longer.