Hey- Glad u found the OS 61SF- These are superb long stroke, pattern motors, and i assure u, that u will be amazed how well ur plane will perform. When u start unboxing, check each of the structure components(wing sections, rudder & stab assy), and make sure everything is up to snuff before u permanently epoxy things together. U can still get ur hands inside the fuse cavity, and add any extra support pieces before u start stuffing tank & other parts in there. Check all the glue joints on formers F1-F4 for proper glue, and add more if u find any lacking. If u feel the upper part of the forward fuselage(between canopy & firewall), and press on it lightly, u will see that will feel a little soft- that's because it is only a thin balsa sheet over a foam core. That's where i added the 3 hardwood stringers between the cockpit and the firewall, as u can see in my article. If u nose the plane in hard, or ground loop it, chances are u might "accordian" that frontal area.I was lucky that mine didn't get squashed, and that is why I didn't mind buying a wrecked one. If u mount ur motor sideways, everything will fit under the cowl nicely, and all u will have to do is cut a slot on the bottom of the cowl for ur pitts dual exhaust pipes to clear the cowl.
As u can see on mine, I didn't want to chop up the cowl with the motor installation, so the side mounted way worked best, and i added the fuel filler valves for fueling and exhaust pressure. BTW- if u can't find a Pitts muffler reasonably, drop me a pm- I have several, and can fix u up with one. Let us know what u think of the plane as ur unboxing all the components, and what u find that u like or dislike about the ARF version-- Dean