finding the parts is not the easiest thing in the world your best bet might be to buy em right form bmw, of course at that point they may not be much cheaper then a forged unit, eagle seems to have pretty reasonable prices usually.
you cant say that there good to 20 lbs because 20 pounds on a 50 trim is not quite the same as 20lbs on a 40r but stock should hold up to around 380whp with a really good tune and some head studs and a gasket, you might want to go slightly taller on the gasket to lower compression but too tall and you will have timing issues.