Check a shop that does custom Harleys. "Off-the-shelf-custom" mirrors come in a wide assortment. Lotsa ways to handle the thread issue...through-bolted, clamp-on, threaded adapter, re-threading the existing mount or the threaded base of the mirror stalk...just have to get creative.
kmcrab
69 CT 70 KO (#650)
70 CT 70 HKO
70 CT 70 HKO
70 CT 70 HKO
71 CT 70 HKO
71 CT 70 KO
71 CT 70 HK0
71 ST 50 DAX (GERMAN)
71 SL 70 KO
72 CT 70 K1
72 CT 70 K1