While a monotherm would be really cool, are pistons a limiting factor in TDI engine performance?
Assuming they are, what about 3d printing (welding) some pistons?
A few places out there that do this kind of thing, typically with more exotic materials like inconel, and such but there are some high strength maraging tool steels available as well if I remember right.
They basically powder the metal and laser weld it in an inert gas environment. It's 100% density and near net shape so all you'd have to do is finish machine the OD, ring lands and wristpin bore.
They're going to be one off parts anyway, right? Production quantities are another story, but at low volumes this might be viable?
http://www.rapidmade.com/3d-printed-materials-resin-thermoplastic-metal-abs-acrylic-aluminum-steel
https://www.stratasysdirect.com/technologies/direct-metal-laser-sintering