i always used industrial sprocket blanks on my old c90...could never find an original part...
having a mate with a cnc mill helped...4 holes and the centre hole, all accurate n concentric, in 5 minutes... blanks cost about $20 and have hardened teeth. while industrial chain, the cheap stuff at least, was useless.
that was for 428 pitch...never tried finding 420, no reason for it then...