Beautiful table Tinpan. I'm not a turntable expert by any means but looking at the pictures, if the arm is bent you probably just want to replace it. You might luck out and find parts, thorens arms show up all the time but it's a crap shoot. If you're looking to upgrade to a fancier arm like a SME, etc, you will have to modify the metal cover. The current one looks specific to the arm. So, it might be as simple as either: 1) being able to modify the hole cut out of the metal cover but it wouldn't look as nice or custom. 2) You'd have to place a new plate cover that has specific holes drilled for your new tonearm.
Again, i'm going by the picture. If this is a suspension table which i'm guessing it is, that's why there is a square hole cut out, you'd have to look to see how it was mounted to the suspension. It is a good project, beautiful table, but it would take some know how. But, if you can find a thorens tonearm replacement, it wouldn't be that bad.