MMM nothing like having a long term goal. Had a bit of experience with aluminium and magnesium. An OHV head is probably the grail in regards to casting. Little spurs to cool valve guides, exhaust water jacket volume, shrinkage and porosity are a few of the hurdles you have ahead. Feeders, runners and risers will be the key to the success of your project. It still amazes me that to cast a decent head nearly 50% of the molten metal is used as feeders to prevent problems with the finished part.

If you are starting from scratch I would suggest you section a couple of iron heads and look around at the layout. Because you will be using a different material you should increase thicknesses in a few critical areas like combustion chamber, block face, bolt bosses and valve spring seats. You will be able to get the metal thinner around things like intake ports and spark plugs to take advantage of the new material.

The 3D printer looks like the go for the pattern in my opinion but for my money if you are going to pay for this to get done I would get a pattern milled out of wood to mould your water jacket from sand. Combustion, ports and spring side will also benefit from this. If it's a hobby project get a pattern makers rule and start carving that foam. You can use foam and parent metal posts inside the mould to support the core if it is a bit flimsy.

With a mould you will then easily be able to produce a few to sort the feed issues and make a good clean weld free part. Using sand will also help with internal mould distortion and foam is way to easily displaced to be accurate. Sand will also allow you to cast it faster guaranteeing a better fill and less leiklyhood of a reject part. It will hold the heat longer allowing the cooling to semi solid to take longer. Certain areas will need to be cooled faster to prevent shrinkage, a good rule of thumb is deep holes need chills and in a perfect world you always cast with the combustion face down as gravity will help get a solid shrink free part. Try to cool the combustion face faster to prevent it from back feeding the rest of the hot casting. A thick steel or iron plate under the box will help a lot.

Heat treatment is easy, once your casting is cool and cleaned out warm it up to 500c from memory and quench it as fast as you can. Hot water is the go as cold will get it harder but will probably have trouble getting around evenly and may leave some softer spots. A water quench will have the added advantage of filling up the micro holes in the casting that will allow it to leak. In a fresh casting without corrosion it is really surprising how porous they can be. After getting wet and left to dry for a couple of weeks the micro leaks will be greatly reduced. There are compounds used by some company's that are vacuum induced into the casting to fill up these holes but that costs money and it doesn't really do any better than letting it sit for a while.

Excuse my ramble I get a bit carried away sometimes. :
Cheers
Daza