The Tach Cable Adapter has 2 seals, an O-Ring on the outside of the Tach Gear Housing, to seal between the housing and the case and an Oil Seal inside the housing, to seal the rotating gear. Try to determine if the oil is coming from between the housing and case or from inside the housing. Both are easy to deal with. Remove the tach cable; then remove the screw threaded into the case that locks the housing in place and withdraw the housing. There's your o-ring. The oil seal is inside the housing under the threaded cable adapter. Unscrew the cable adapter from the housing and there's your oil seal.
Try as Yamaman suggested and re-torque the cylinder head nuts before resorting to removal of the engine to replace the cylinder base and head gaskets. If you do end up replacing these gaskets, consider replacing the stock steel/rubber head washers with copper ones to provide a more even torque load to the head. In fact, consider fitting these even if you do not end up replacing the gaskets. They can be easily fitted and also provide a great seal against oil leaks.
The areas INXS pointed out are common leakage areas to watch and address, if necessary. If you replace the clutch pushrod seal, check the bushing there for excessive wear and replace, if necessary. This can all be done without removing and splitting the engine, but care is necessary when doing it. Also consider a one-piece push rod here to reduce wear of the bushing and resultant oil leaks. If the seal leaks behind the sprocket, there is a kit available to allow you to replace this seal without engine removal.
Have Fun!