Good replacement actuator?
Something that meets or slightly exceeds the 35 in-lb rating, and matches the 24V spec (though you could check if your system is using AC or DC, and just get one to match your system rather than an AC/DC version.)

I suppose the time should fall in the general range of this one's 80-110 seconds as well, so vents are not slamming open and closed, but I suspect that is somewhat less critical. The power draw should not be too much more than this one at 3VA or you may overload the system that's running it.