You could use something like a Schleich eagle for vulture for a roc.
For the giant, the description is interesting:
"... a huge creature in the likeness of a man, black of colour, ... with eyes like coals of fire and large canine teeth like boar's tusks and a vast big gape like the mouth of a well. Moreover, he had long loose lips like camel's, hanging down upon his breast, and ears like two Jarms falling over his shoulder-blades, and the nails of his hands were like the claws of a lion."
I reckon any particularly ugly troll, ogre or giant would do the trick here - the key ingredients seem to be tusks, clawed hands and slack lips (you could dismiss the "upon his breast" bit as hyperbole). Otherwise, the main thing would be to paint him black with red eyes.