I am looking for advice on material selection.  I am designing a one-off prototype aluminum cylinder head for a 2.4 litre, liquid cooled 4-cylinder inline four stroke engine with pushrods and two valves per cylinder.  The OEM cylinder head is cast iron. The OEM engine block is cast iron, as well, and has wet sleeves.  No nitrous/turbo/blower operation is planned.  The idea is to have the head machined from billet aluminum.  I am planning to copy major dimensions from the original head and improve on the ports and combustion chambers. 
What would be a suitable alloy for this application?  Availability and cost are of course factors.  What are some of the not so obvious things to consider on this project? 

Ive looked into this and as Pat said, water ways are the problem, by the time you have made yourself enough tool access points in order to remove sufficient material around the combustion chamber you have weakened the head alot.
If you skimp on machining then you end up with hot spots which as you know is'ent good either.
I take it your using alloy for its improved heat transfer properties over cast Iron?filter
Do you intend bumping the compression ratio to take advantage of this?

If you are going ahead and have worked out a milling plan that your happy with in order to give you adequate cooling above and around the combustion chamber then you need to get some 6061 billet, it welds well too which may be of benefit to you as I feel you will need to close up/plate any mill access points which, when finished will end up as flange faces etc...

You will also need to take it to a t6 temper, Im sure your aware of that.

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