Replace an Alvit? I would have said "yes" in 1973 and I still say "yes" now, assuming the bike is for riding.
If it is for hanging on a wall as a 100% accurate restoration to be exactly as the bike was when new and it had the misfortune to have an Alvit hanging on it when new, then keep the Alvit. Of course, if that was your mindset, you wouldn't be asking, now would you?
As for the old plastic Simplex RD's, there's a reason why more of them are in parts buckets in old barns than out on the road racking up miles . . . .