I installed a seuthe HO unit into my chloe.
I used the one which has a larger bottom than top, and i cannot recall the number but i believe it is unqiue in its shape.
It has a surprisingly good reservoir.
Otoh, it has such a small opening that the use of a hypodermic like syringe (supplied with the unit) is essential, otherwise surface tension keeps the smoke fluid on top.
Heres what i know.
First drilling the stack is a must, and you should check out how they connect into the body
since replacements may be difficult.
Also be aware that smoke fluid overflow will go into the area below.
You may only need a hole wide enough to accomodate the wires, depending on the how the unit itself fits into the stack. This wasnt an issue for the Chloe which has a balloon style stack.
As I recall, I believe the FRR stacks simply screw into the boiler, so drilling shouldnt be an issue. The US versions have an odd sort of hook like arrangement.
Second, since I presume you will use track power-be aware that even the 16 v HO units require that the loco run rather quickly to produce more than a wisp.
I would check the length of smoke unit you choose along with its girth, as the FRR straight stack is quite narrow. The larger turban style might be more accomodating if you can locate one, and, you will have a spare.
gratuitous pics to follow: