I'm repairing and resealing a neighbor's '88 3.0L Cobra. Marineengine dot com has full parts diagrams for OMC drives. They stock available OMC parts as well as Sierra parts which cover most NLA OEM parts. I copy their p/ns and see if they are available here at iBoats or google for better prices elsewhere.
'89 and earlier 7.4 Cobras used 0915850 OMC gasket, 18-2918 Sierra gasket. It appears '90 later model Cobras used a formed O ring in the OMC 3850755 upper gear case seal kit.
I hope you have an earlier model. The gaskets are widely available. I couldn't find a later model gasket kit. Good luck.