Boy, that would be an interesting project, I think Kirkham is all set to machine a prototype for you. I think people have used magnesium for blocks in the sixties on occasion. .06 lb/in3 Aluminum is .1 lb/in3 and Iron is .3 lb/in3. Titanium has issues beyond machining. It frets like crazy, so it would be a pain to be constantly rebuilding, pressing in liners, etc. Titanium is great when you can't get past the fatigue or temperature limits of aluminum. So on connecting rods, rocker arms, spring retainers, even springs it can be an expensive but necessary material. I would love to build an engine with titanium valves, rockers and rods, but in general I'd never get the benefits beyond bragging rights
With an engine block, in a lot of spots you need minumum material thickness, which probably relegates the idea of using titanium to the way-back of the list because the ultimate weight savings would probably not be there.
The price of titanium is huge right now also, I think the block of material before it's machined would be $15k or more. You could make a cast titanium block for a lot less I supsect in a volume run of blocks.
Lately I've seen Aluminum-Magnesium Rocker arms which seems like a "newer" exotic material that might have weight and strength advantages over other aluminum alloys, at least in what I read.