Bath Sink Drain (50 yrs old)

Hi,

I belong to a little 50 year-old brick bungalow in the midwest.

There's a small bathroom that's never been remodeled. A small
sink with a U-trap, drain running horizontally thru
tiles into the wall maybe 14" above the floor.

The portion of the drain that runs from the pvc U-trap
into the wall is badly rusted. The drain runs very slowly.
I've cleaned out the U-trap, run Drano thru, etc (no help).

I can't see anything of the sink drain from the basement.
I figger it does a bend and runs (inside the wall and
floorboards) into the toilet drain. But what do I know? :-)

At least part of the sink drain pipe will need to be replaced
before long. I am sorta half-handy at plumbing. How might
such replacement be accomplished?

Thanks,
Peetie