Celestron Nexstar 102GT
The manual says the original eyepieces had 1¼ inch barrels and focal lengths of 25 mm, 20 mm, 12.5 mm, 8 mm (all reasonable) and 4 mm (not so much).
If I had to pick two replacements for a beginner, I'd get a 25 mm and a 10 mm.
You can probably get an entry-level Plössl by Meade, Celestron, or Orion for US \$20-25 from the Cloudy Nights classifieds or \$30-40 new.