Getting back to the OP, I would target the iPad only, and skip trying to stuff things into the iPhone. If the game is 800x600, then this would leave room for other interface options in addition to just the touch gestures. The iPad also has a physical keyboard (but I would make it optional if at all possible).
Software/Technical: Depends a lot on the original development environment, and whether or not it used the OpenGL library (for the Mac OS version). I have no idea what Spiderweb uses now, but (I'm guessing here based on the age of the product line) the mostly straightforward (note I did not say easy) scenario would be if the current "game engine" is C based, uses some flavor of OpenGL, and is NOT based on the older CodeWarrior environment. Anything else will be problematic and of increasing difficulty. (If someone wants a more detailed opinion, PM me).
But these concerns all take second seat to the business ones. In my experience, game companies that can leverage an existing code base have a leg up on folks. If you have based your design to some common code base, then the cost for porting is divided over multiple products, and this is another plus. Looking over Spiderweb game library, I think that a price point between $5-$10 is possible and reasonable -- more than that will be a tough sell to the the target crowd I think. This is less an issue for the older games than for the newer games in terms of recouping your investment, as it may cannibalize sales.
Just my $0.02
- Bunshichi (an iPhone/iPad/Mac developer, with both itunes store and enterprise apps)