Tom
I have the #121 electric from Mr. TOM now (excellent runner and love it) I was just looking at PM research for you, they have a flywheel made for they're #1 steam engine ,part # FWC-1CI it is a little bigger on the od side @ 6.5"OD x 3/8" wide with a 3/4" hub.but it is 5 spoke.
I'll check it out Bruce, thanks.