Is it possible to create a mold of an aluminium motorcycle cylinder head so that you can create an exact replica with melted aluminium?
Yes it is but there a a few things you need to watch out for.
You will need to allow enough excess material for any surfaces which need to be machined subsequent to casting.
All castings shrink somewhat so the pattern needs to be a some percentage larger than the finished casting needs to be.
Its not uncommon for castings to have internal voids which are created with cores in the mould these are a bit more difficult to reverse engineer than the exterior.
Finally you need to add the gating (runners and risers) to the pattern. Getting this right is very important as it not only lets the metal in and the air out of the mould, ensuring it fills properly but also controls the cooling rate, which if you don't get it right can cause shrinkage voids, hot tears and other defects.
Cast aluminium is also quite prone to porosity and ideally needs to be degassed if you want to make decent quality mechanical parts.