I’ll tell you what, guys. Mounting on the side of the frame with a piston engine is a bad place to be. Works on electrics. But you have to deal with the back and forth accelerations of a single-cylinder piston engine. Every time the cylinder fires the piston accelerates one direction, the crankcase accelerates the other direction.
The rotating mass in the crank counterweights and flywheel has to match reciprocating mass in the piston and rod. And this is another story. I run balanced 2mm stroker engines that have been dynamically balanced to run at 11,500 rpm vibration-free. Besides a 2mm stroker crank, they have lightened piston and rod, and due to the stroke they have higher compression ratio than a stock engine so they can’t run 87 octane pump gas. You can’t achieve secondary (non-sinusoidal) balance with a single-cylinder engine, so it only has a narrow rpm range where it is designed to run.
Running stock engines will always be problematic because the RC-format engines were designed for racing buggies, not helicopters. They have to rebuilt as a helicopter engine, new out-of-the-box.
I have my flight control in one helicopter mounted in pretty much the same location as shown in this thread, with zero problems with vibration. It is mounted right directly above the engine’s cylinder and has flown over 80 hours that way - with the cowling on it which encloses everything pretty tightly. Although there is some air ducting designed into the cowling for engine cooling:
If you have a stock RC-format engine you will never get acceptable vibration results from it no matter where you mount the controller.