The American flag rifles don't take AICS mags. They use proprietary Howa mags. I can't tell for sure but it looks like the removable bottom metal in the flag chassis might be able to be replaced by a PTG unit.
I used a clone of the South African "Gun Warrior" chassis that a buddy in South Africa that makes custom chassis stocks cut for me, then added PTG bottom metal and Accurate Mag AICS pattern mag, a new barrel from Columbia River Arms custom chambered in 6XC with a Tubb spec reamer, a McGowan barrel nut and a Timney trigger and JP brake and now it's a shootable rifle.
As a matter of fact since you're looking at Howa, this rifle (which is my backup rifle for match work so it's only got a couple hundred rounds through it) is up for sale. With the US Optics SN3 3.8-22x44 and the Harris bipod it's $3000. Without the optic/rings/bipod $1500. When you look at the cost of a Flag gun...