Need help identifying a problem, Engine Leak

When I bought this car, it had a leak. Knowing this, I got under the car to wipe the engine down, with the intention to go driving and see where the leak was coming from. When I was done wiping the engine down, I noticed this:

Spoiler

The dent/hole is up in the top left, with the oil smeared around it.

I would like some help with identifying what is the problem here and, if needed, the part I need to replace.
The engine still holds its oil and drives fine, just have to top it up every now and again.
I think the issue is a leaking rear main seal but would still like a bit of help, thank you.
*edit* Forgot to add that I have a bit of a weird car, it is a 2005 Mazda 3 HB with the engine out of a 2007 speed 3, hence the name 05Speed3.

 

Rear main seal leaking that much "should" mess up the clutch but don't know what else it could be

 

When you say "mess up the clutch" should I be feeling a slippage of some sort or something else?

 

I would expect the clutch to start slipping with that much oil coming out of the rear main seal