Here are a couple of pics of an OS 75 AX install. I used a GP mount and centered it with the centerlines on the firewall. I installed a Slimline pitts exhaust shown in pics. There is enough room to allow a remote glow but I chose to just drill a hole for the igniter. I have not performed a CG check yet but should be able this weekend. I had this engine in a GP revolver that met it's demise already. No CG problems with it at all. These planes are similar. The Sundowner 50 has a smaller chord on the wing and smaller control surfaces. I think it is suited for speed much more than the Revolver which was a great sport plane. I did have to cut out about half the cowl on it to fit the 75AX though.