While I'm NOT familiar with American Arms, if they're still an operational company, I'd contact them and see if there are any popular brands of Choke that are available, that would fit properly. Tomorrow is Monday, and they should be open, if they're operational.
Or, I'd contact a 'Smith and ask.
But, with the amount of Experience of alot of those on this Forum, You may get a solid, timely answer. Good Luck.