Assuming you do have a wye on the line, not a santee, then yes you can use that line without digging down and replacing the fitting.
An 1/8 bend will let you either go vertical or horizontal.
If the angle is not quite right, you might use a long sweep 90 el on it's side and tilt it where you need it.
If that was the only cleanout, you can add a new one under the lav.
I know that you will vent the toilet and the lav when you run the rest of the plumbing.
Otherwise when the other plumbing is used, it will want to either push or pull water from the basement fixtures.