As a point of reference, "head tube replacement" on a titanium frame is listed at $600 on Bilenky's website: Bilenky Cycle Works | Repairs. This seems more or less in line with what I'd expect from other builders willing to take on the challenge. If you really like everything about the frame except you want to use a modern fork with it, I'd say it's worth the investment, but if it's just any old titanium frame for you, you can find titanium frames on ebay that already take a 1 1/8 or tapered fork for less than that.