RE: Pilot R/C, 37% (100cc) Edge 540
Since we're on the subject of engines I'll bring up a tidbit that will ease engine mounting.
The crankshaft centerline falls in the exact center of the mounting plate, making mounting template construction real simple. All the mounting leg holes are equidistant from the crankshaft centerline, at roughly 2.522" crank center to mounting hole center. Distance between mounting holes is roughly 3.545".
I'll make up a template and post pics in here when it's done. I'll strongly suggest making a template before mounting the engine to assist central or offset engine placement and promote easier trimming work for the intake manifold and carb. A simple square ply plate with a 3/8" wood dowel extending out from the center will be all that's required.
One could, if they foolishly desired, mount the engine flat to a firewall without any standoffs at all but that would be havoc with cylinder cooling. At least an inch, perhaps a little more, should be allowed for.
I haven't done an assembly thread in a long time. Forgot how much detail goes into one. Now the hard part starts. I need to get into the assembly phase and associated documentation. Pardon me while I take a breath for a couple days before starting that part.