Hi again guys, this P51-D I've accquired needs re-powering, prior power - OS45 MAX, 57" span built from kit so is solid. Suggestions Please.
Thanks in advance.

Maybe an OS 46?

How about some details? Make, weight, how it flew with the last engine, etc.

Like Charlie said, more info is needed. We need to match the power of the engine to the capability and weight of the airframe.
A .45 two stroke will make a lot more power than a .45 four stroke.

Just a guess at 57", It would probably take a decent .60 2 stroke or a .80 stroke to make it perform.