So I had a chance to put some tires into the frame and see whatll fit. Sadly to say, my 2.2s cleared the chain stays and sesttube just fine, but it rubbed on both chain stays. That may be from the fact the rim isn't fully true but even without the minor variation in it, I wouldn't recommend to anyone. The 50 cleared the front with around 6mm on either side so 2.2 would be fine up front.
I will say that 2.2 race King looked damn good in the frame, if you shave the side knobs off it may work but kinda seems like a waste when you could get a proper 2.1, just watch the side knob height.
Here's an album of photos I've taken so far. No calipers still so no solid numbers but the Allen key in the photos are all 6mm wide. Rear tire is a 45 terra trail and a 50mm
https://photos.app.goo.gl/mfz5rBSy52Wv4Z337