The Volkswagen 1.9 TDI rotary-pump engine has a fair amount of stuff available for it, if you know where to look. In USA, these were sold 1996 through 2003 without major changes. Try
Fred's TDI Page. TDIClub.com. VW TDI Enthusiast Community for the goods on that one.
The successor to that was the "pumpe-duse" (unit-injector) 1.9, of which I have one example. Mine is chipped. Works well enough for what I do with it. The version here in North America is still a 2-valve-per-cylinder design, but the fuel injection system is different.
The real trickery will be with the 2009 engine. Common-rail injection, 4 valve DOHC head, 16.5:1 compression, and a TON of emission control stuff.
Agreed that there's a fair bit of room to improve the TDI engines. The intake port is typical production-engine rough cast. The exhaust ports in the head don't even come close to matching the holes in the manifold. The camshaft is incredibly conservative. The turbo is pretty small (and pretty weak) in the interest of low-RPM takeoff from a standstill. Neither the intake nor exhaust manifolds are particularly stellar design examples ... they're designed to be cheap and to get the job done. When starting with the deliberately de-tuned lower-powered models, it's almost a no-brainer to bump power output by 50% and some people have doubled it. Beyond that it starts getting tough (and you'll be well into needing stronger driveline parts anyway). The factory made as much as 150 hp from a 1.9 P-D and 170 hp from a 2.0 (four valve per cyl) P-D. A pretty good starting point is to just copy what the factory did on those ...
Here is a TDI performance bits supplier to check out:
KERMATDI Home Page