Cheif aircraft sells custom standoff mounts. I have one and it looks great [link=http://www.chiefaircraft.com/cgi-bin/rcm/hazel.cgi?action=serve&item=/Models/Model.html]cheif aircraft[/link]
the stock muffler is a good choice but if you have the extra $$ opt for the mtw or ks 75cc cans. They provide more power and are way quieter.
Your second point tells me you are new to a gas motor. That center hole is threaded for the bolt that holds the spinner. Do not try to put a "contraption that will fix that problem" in that hole.
The back plate issue is simple. From the engine forward you have the output shaft, then the spinner backplate, next install the prop followed by the drive washer. Once the bolts are tight you can mount the spinner with the bolt that comes with it into the center of the shaft. I hope this helps and if you still have questions I would head out to the field or hit a local hobby shop for a hands on answer.