Alwood, your first drive sprocket - the one on the engine/clutch shaft - is completely worn out, which may mean that the first chain is worn. You should remove that chain and hold it horizontally on edge, observing how much it sags. The sag is due to wear between the pins and sideplates, and it increases the pitch of the chain, which destroys the sprockets. The other sprockets that I can see in your pictures look good. You just need to replace the clutch shaft one, and you may or may not need to replace that chain.
The chain tensioner on that first chain is positioned on the slack side of the chain. The drive occurs by pulling on the other side of the chain - note that the top (clutch) sprocket rotates anticlockwise when viewed from the chaincase end. It does not matter whether the tensioner presses directly inward, or slightly upward or downward, as long as it is on the slack side of the chain. Trying to tension a chain from the tight side would be incorrect design of the chain drive.
So far as the oil leak is concerned, I haven't yet figured out what part of the engine your picture shows. Some more explanation or a picture from slightly further away might help. You might take a look at this thread though:
https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=37772#Post37772
