For a 40‑size Hanger 9 Mustang that originally took an OS 46, a good electric replacement is an outrunner in the 40–46 size range, around 900–1100 kV, paired with a 4S or 6S LiPo; the original manual for the P‑51D Mustang Sport 40 recommends the E‑flite Power 46 BL outrunner (about 900–1000 kV) with a 60 A ESC that has a built‑in switching BEC, and a 4S 3850–4500 mAh battery, so a similar setup (same motor size, 50–60 A ESC with a 5–6 V switching BEC, and 4S or 6S pack) will give you familiar performance and plenty of power for sport flying.
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