Hi Jeff,
I don't know if my N42s have a complete set of valves so may need to source more as p90 valve stems are longer than N42 so can't be directly swapped.
I also took a camshaft out for my E31 head (internally oiled) swapped for the external spray bar set up on early cars.
Re: comp ratios if I recall correctly flat top pistons with an N42 head yields a 10:1 compression ratio.
There was an online L-series engine calculator years ago on Ozdat.com but I think it died.
You could try this though!
http://atlanticz.ca/zclub/techtips/enginemodellingsoftware/index.html
However it's not entirely accurate so use it as a reference point not the gospel.
I was just down your way yesterday, should have dropped by with the heads ?