Looking at the pictures on your link, I would scrap that frame, if the fork failed due to that much corrosion, the rest of the frame probably has issues as well; snaping a rear dropout as well would give me no confidence in the frame.

For checking corosion, use mark one eyeball, if somthing looks corroded, see how deep it is by scraping / wire brushing off, if it's internal, the you could X-Ray / endoscope, but that would cost more that the bike is worth.

If any of the frame needs repair, the cost of getting a frame builder to replace the tubes will be more than the cost of a new frame