I'm with MJ on this one,it's gonna be a thriller! Don't believe anything you read by forum experts to be gospel P90 valves are shorter as the chamber is deeper in the head. Comp ratios are fickle which rely on accurate measurements of piston to deck height,chamber volume, head gasket thickness and bore diameter plus any valve reliefs in piston and so on. Machine shops exist for a reason, they are tradesmen with years of experience and are in business to do the correct job but also to make money. Put an N42 on it with a bigger cam and no valve reliefs and you will die, and be back at the machine shop to do it all again. I don't set my house on fire just to have a go at putting it out myself.