It would be a very large undertaking and I'm not sure it would buy you that much if it worked. Remember this series has a known issue with the GPU so you can't push it that hard without it failing.
While the A1226 is able to run newer OS's than the A1211 the performance is not that good and many apps today need much more memory than the limit of 6 GB you have here. I think you can do better looking for a newer used system than trying to get this system working again
Just to be clear here OS-X El Capitan 10.11.x is the newest you can run.
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