Peter,
Coming from my limited knowledge, here is some food for thought on the Koa Isshin. First, go to R. Stein's site and read his article on the Koa Isshin blades. For that matter, read the whole site. The article indicates that a good example form this group of blades would have some definite value. I know of a few instances of martial artisits using these blades for Iaido and Battodo and being very happy with them. I own one and it is a beautiful, well made sword and one of my favorite gendaito. That being said, the issue of whether or not it is worth restoring has alot to do with it's current condition and what you want to achieve. I believe these blades are going for $1,500 on the mid-low end to maybe $2,500 if it is in excellent condition with the original koshirae. You could spend at least $1,250 for a decent polish if the blade is not too badly rusted. Then you have mounting costs if you wish to use it for practical use. So is it worth purchasing? Possibly, but you'll need to provide images of the full blade before that can be even guessed at.
Thanks,
Kevin P.