I read somewhere that JT Racing makes an adapter for Hyper 7 diffs that allows you to use the Ofna 51t spur gear. It would fit the Hyper 8 diff, so it seems like that's what you're looking for.
SH Z-Car, Custom Crawler, 8s Savage, 12s XTM XLB 1/7 buggy, 4wd 4-link rear/IFS Pro4 truck, Custom Hyper 10 Short Course, Belt-Drive Mammoth ST 1/8 truggy, 4s 17.5 MM Pro HPI Blitz