Almost EVERY experienced performance engine builder worth their weight in beans will advise to start with a brand new engine case when assembling a performance build.  Most will recommend using a magnesium case, unless all-out racing and RPM's over about 7000 are desired.

This starts as a factory VW replacement case, made in Brazil.  The cylinder bore openings are machined out to accept 90.5mm or 92mm cylinders, so popular engine combinations like 1776cc, 1835cc, 1904cc, or 1968cc engines can be assembled with no additional clearance needed.  94mm cylinders can also be used if additional machining is performed on the cylinders themselves.  In addition, case savers are installed that utilize 8mm head studs for best performance on street-driven engines, as they provide the proper amount of stretch and sealing capability. #3 head stud is "deep sunk" to help prevent cracking of the web at the bell housing.  The case is also machined for "full flow" oiling, which provides for a fitting to be installed so you can route your oil to an external oil filter and/or oil cooler, if desired.