Engine Block Material
 

Anyone know what material the engine block is made out of of? I removed the oil galley plug and it took some of the threads with it. What should I do, weld the existing plug on or retap and get a larger adapter?

Aluminum alloy. The accepted method of repairing stripped aluminum threads is helicoil. Given how critical that oil gallery plug probably is I'd seek advice from a professional machine shop familiar with rebuilding Subaru engines if I were you.