Spark Plug for 4AGE BT 20v engine is IK20.
For further information, u may refer this website
http://www.globaldenso.com/en/products/aftermarket/plug/basic_knowledge/heatrange/index.html
what i know from friends is that the VJ23 are considered quite big turbo... but just wanna find out more... if using a 1.8 engine what bigger turbo can be used without increasing too much lag??