Was looking to go down this path also. A Cosworth 2.6L stroker for the 2.5 wasn't going to leave much change out of $20k after parts, labour, r+r motor, etc etc etc...............etc (list goes on). Going down the built 2L (Ie. 2.2L) path was a similar story.
In all honesty, I'm glad I never built the motor. It does the odd few track days and gets a good thrash on the road and still running strong (touch wood). In all seriousness, is 0.1 - 0.2L really worth the trouble? Apparently you wont rev as much either and you will be refreshing the motor every X0 000 km.
Seems like a whole lot of wank/bragging rights unless you're super keen on a race car. My 1.8c