Can i run a P90 or N42 with dished pistiions? These are better flowing heads than the N47 correct?

Can you run a P90 or N42 with dished pistons? Yes.

Are those two better flowing heads than the N47? Definitely for the P90, maybe for the N42. But in both cases the flow improvement might not be noticeable without a lot of other mods.

The big issue is that neither of those heads will give you much compression with dished pistons. The N42 is probably the better choice, it will give a CR in the low 8s, or about stock for an L28. The P90 with dished will only give around 7.5:1 or so - fine for a turbo, but lousy for a naturally aspirated L28.

I'm going to assume that budget is an issue, otherwise you'd probably be looking at flat-top pistons instead. If that's the case, I'd skip the head change and put your money into a good valve job and cam for your N47. There's lots of good running motors with those "poor-flowing" round port heads on them.