For most of the generators I have the original field rheostats.
As the AVR's are connected in series with those, so I don't expect too much trouble. They don't need to adjust over a wide range, since most of the generators we have are way too big for any load I can think of. (35, 40, 60 and 125 kVA).
You don't want that Isenthal rotary AVR. It's not gonna do you any good. Just give it to me and forget about it.
ps: that is very high priority on my wish-list!
Hubert