Clean the rust off a couple of shanks and you should find either a number or a letter stamped on the shank. That is the style of shank you need. Sometimes its also stamped on the side of the saw, above the eye. Most common are a B, D, F, 2 1/2, 3, 3 1/2. Anything else will be a challenge to get shanks or teeth.
If its hard to see, you might be able to do a tracing to get it.