If it has no screw in choke , your best bet is the the federal flight control wad , on either the lead or heavyweight versions - with luck you may be able to make it a 40 yard gun, most Ithaca's came with a full choke
You could have the barrel cut for chokes , William from sumtoy might be able to do it
Or you could try and find a threaded barrel for it on eBay , I've seen them before and the chokes that come with them are a very obscure tread pattern , again you will need a custom choke made ...
Towards the mid 1980's Ithaca made some shorter barrel Ithaca mag10's with screw in chokes , I remember Dick Kirby of Quaker boy game calls had one and it was a real killer , Ben Lee also shot a Ithaca mag10
If I could find a 26 inch mag 10 , and get it cut for remchoke , I would be very tempted to buy it , that's on my bucket list along with getting a Winchester model 12 3inch , cut down and choked ....