You need to add a breather onto the back of the crankcase. I assume the breather hole on the front is blocked? 40mm core plug or similar behind the exhaust manifold?
Options are:
Re-instate the OE breather system and use a different exhaust manifold (adapted 200sx one or similar)
or, and cheapest to do
Drill a hole in the back of the block and plumb in a breather.
Ideally you want the engine to be breathing from the block and the head into a catch tank. That can either be drained back to the sump or you can empty it ( Better not returning it to the sump IMO as it'll be dirty oil and you dont want that being pumped through your expensive turbo!)
Theres loads of info on here in various build threads, of have a look on the quantums site. That gives a detailed step by step ZVH build and where he's added additional breather ports etc.
http://www.jimhearne.co.uk/zvh.htm

Good Luck!