On the racer, I built a small steel flywheel just big enough to fit up a 5 1/4" Tilton dual disc clutch pack. The front side needed to have about 1/4" spigot so as to clear the flex plate which has a slight conical shape. I just bolted the flywheel on over the flex plate. Never a problem on the 95 RX-7. a big problem on a sports racer I built. The driver defeated the rev limiter by shifting into 1st rather than 3rd while already turning about 9,500 RPM. The transmission was a Hewland FT-200, dog ring style, so it went right into the selected gear. The flex plate has only 6 welds to keep the ring gear on the flex plate. The ring gear broke next to one weld. The free end tried to machine the bell housing into 2 pieces. The ring gear also rolled outboard about 30 degrees. It wanted out and almost made it.
On the replacement ring gear I TIG welded an additional 6 welds and added 12 welds to the back of the ring gear.
Since you do not need a wear surface for the airplane installation, just a piece of aluminum plate could be used to produce a similar flywheel. The flex plate is just used to start the engine and delivers no torque loads to the gear box.
Lynn E. Hanover