Iused 6cm standoffs (sorry but I am going metric) for the engine, the distance from the rear of the cowl cover where it meets the fuselage to the firewall is approx 11cm, and the distance from the front of the firewall to the back of the spinner is approx 23cm. Note the cutout for the silencer.
I also fitted the 3s lipo needed for the brushless starter motor above the engine on a removable cradle (also use this to power the ignition). The ESC sits on the shelf projecting from the firewall on the LHS.
I cut the cowl so that the upper section can be easily removed with a single catch for access to the battery. Also makes access to the engine and carb much easier.
The starter is beautifully engineered and works just like a bendix car starter motor. The ignition and ESC has some clever programming like revesring the starter motor when the engine starts or starter is disengaged so that the bendix gears disconnect cleanly. And all controllable from your transmitter.
A favourite trick is to taxi out, stop the engine in the middle of the runway, wait for everyone to groan and point a finger at my engine setting up skills, than amazingly the engine restarts (with simulated smoke from the onboard smoke pump) and away we go!!