If someone wanted to build a sporter off an Arisaka with a partially filed off mum, would anyone restore the metal and engrave a new mum?

I realize it might be considered "faking" original markings, and if so, what about doing it with a little artistic license?

Just wondering. I was offered an Arisaka in rough shape.

Firstly, I would not use an Arasaka action but if you really wanted to do it, sure, go for it. Grind off the receiver to make it round and then engrave away. I would not weld up the place where they defaced the mum though. Welding on receiver rings is usually bad form and the Japanese used some very tough steel for them.