Replacement Motor for a Vintage Thorens TD 160 Turntable

Motors don't come apart.

Indeed.
You have to take them apart.

Slipping? Are we sure it's the motor and not the belt?

My son has done enough checking to isolate the problem to the motor.  So I ask, is it possible to buy a new or rebuilt motor?  Or, are there any shops that can rebuild the motor?

If not, I surmise that the TT is scrap. 

The belt is OK. 

Try this place: 
KAB Electro Acoustics
Preserving The Sounds Of A Lifetime
www.kabusa.com
www.twitter.com/kabelectro
P.O.Box 2922
Plainfield, NJ 07062
908-754-1479 Phone
908-222-3442 Fax

It would help if your son could post the symptoms (in detail) and what he has checked so far here.

Having owned every version of the 160 except the Super "motor slipping" doesn't ring a bell.

Has he checked the larger capacitor?

To answer your OP there are motors currently available on eBay in the $250-$300 range (supposedly NOS, but no guarantee).

DeKay