Just saw this pop up. The original art status is unknown. I don't know if the artist(s) were identified, or if they got any residuals. They may have been work-for-hire. If so, they would be PD in Canada, but their status is very gray.
I would recommend building your own covers and leaving off the internal illustrations.
GA, if you want a sample, go to Project Gutenberg Australia and look up Laurence Donovan. Not the best Doc Ghost, but not the worst.
Another proud Tarnover graduate!
Remember, no matter what they say, people don't want the truth, just their prejudices reinforced. -RSE