OK. so normally I wouldn't try and remove the throttle plates, but now you know for next time.
1. maybe you can drill and tap the throttle shaft and use new screws. I wonder why the screws sheared off. If you look at the other side of the screw/shaft (non-head side) you might see that the screws are staked in do they don't come out...If you go this route, you will need to make sure the new screws don't easily come out either.
2. you might be able to get replacement parts-used. Maybe one of the vendors that rebuilds carbs has some parts or check the parts for sale section. The throttle shaft and plate doesn't come up as a OEM replacement part. Neither does the throttle assembly.
3. Buy a used carburetor assembly and start over. You can use the best parts from either carb to build one.
I actually had success drilling out and re-tapping after I broke a screw on my choke shaft. It was pretty tedious but I was able to fix it. Replacement screws were sourced at an oddball hardware store.
I will upload a picture of the tap and anything else related later tonight.
Use a thin file to cut a slot in the staked side of the screw and "tighten" it out of the hole.
As far as replacing the screws, word is they are NLA, but FJ40jim and a few others have detailed how to secure the butterfly with some other hardware.
You'll have to try some different search combo's, but it's in here.
The screws break off because they are staked to the shaft to keep them from accidentally unscrewing and being swallowed by the engine (Although the tiny soft screw would most likely do little to no damage).
Those screws are a very odd-ball size (posted somewhere by Jim C.), and thus, hard to find. Staked for a reason. FSM has a procedure for removing and reinstalling (re-staking), if you have to. Might be easier to just find a carb core and use the throttle body.