If access to the core plug is available, it can be replaced without engine removal.
If access is unavailable, engine must be raised or removed from hull.
Most likely, from your previous description as to location, it is one of the core plugs located behind the flywheel....
The engine will have to come out to acess it.....
You should pressure test the block to see if there are any cracks.....
You might be lucky & it is just a plug....
Good luck!......
