Titanium is no good for engine stuff in general. First, the most common Titanium (TI6AL4V) wholesale is $130/lb for the stuff, unmachined. The machining process WILL cost thousands of dollars, assuming you can FIND someone who will work on it. Titanium is some 10 times stronger in a soft form than work hardened stainless steel. This means it requires diamond blades and other exotic machining processes. Next, it work hardens INCREDIBLY fast, and will become 100 times stronger than stainless steel when you start working on machining it.
If that still doesn't turn you away from titanium, then the this fact will: Above 500 degrees celsius (an engine gets hotter than this) titanium begins to absorb HYDROGEN from anything around it: metals, oil, gas, air. It does the same thing for oxygen and nitrogen and carbon above 700 degrees celsius. This will cause nearby metals to weaken ALOT and break.
Sure, titanium makes a good tennis racket, but anything at high temperatures and it doesn't work so well.