The GZ .061 spinner is a masterpiece of machine work, and I would think is adaptable to other 1/2A engines. I've never had any luck with plastic spinners on 1/2A engines. Anything but running perfection will rob gobs of power.

Andrew, depends on the application. Sprint cars dont use a flywheel. Some drag mobiles used to run a heavy wheel to keep the "Rs" up right after dumping the clutch. This thinking might be considered "old hat", but those pictures that you used to see of a '68 Camaro resting on its' back bumper probably had something to do with a 40 pound flywheel spinning at 7500 meeting the back half of the [soon to be woke up] drive train.