I want to make some modelling chocolate (Toba's recipe) but I only have regular corn syrup and for the small amount I would be making I don't really want to buy light corn syrup. Would regular corn syrup be ok or would it affect the final product?

By "regular corn syrup", do you mean dark? If so, I don't see why it wouldn't work. It would give a slightly different flavor, but I think the consistency of dark and light corn syrup is about the same. I have light, golden, and dark, and they all seem to be about the same viscosity - just a more noticeable taste as the color darkens. Chocolate is dark enough that the color in the syrup won't matter. Anyone else with experience here?

I don't know, but I think that ibmoser is probably correct. There would be a difference in the flavor and the color (depending on what color chocolate you are using), but I think the consistency would be fine.