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.