I'm not sure. I think you can pretty much get away with stock throttle setup on that mount, perhaps I'm wrong. The new era mount raised the engine up an inch so you had to deal with that by using a wire guide block and a long kinked wire to get up to the throttle pivot.
On my 21TM revo, I just...
Maybe send THS an email to see if they could tell you the ID of the pipe end of their header:
https://thsracing.com/shop?keywords=maxx&olsPage=products%2F145hm-t-maxx-hard-coated-manifold
Ha! Actually, just noticed, they post the size on the page, looks like they could use spell check though...