Hi GG,
As for the tappets - If you have already adjusted them then they must be manual adjust not automatic adjust as i expected. - That said Manual ones should not need to be replaced unless they are damaged, they do not really wear to a point that they can not be adjusted by screw, shim or bucket (depending on the design).
With Automatic tappets - oil filled, they can normally be stripped, cleaned or all oil, decoked and refitted.
As for the oil leak, it would seem that there is a lot, is there traces inside the Cam Cover at the top or only the bottom?
It its at the top also, could be a rocker cover, cam shaft seal, possibly a head gasket (depends on where the oil gallery is located) or the sump gasket.
I am unsure if there is a Gasket as such, but one way or the other there will be a seal, whether its moulded to the sump, a press in place gasket on the sump, a metal frame gasket or RTV (silicone) there has to be some form of seal.
As you have oil both sides of the engine, it must be a large leak, pressure wash it off, and look again after a short drive to see if you can locate the culprate.
Is there any other strange noises, or are you down on power?
Ciao
Simon