Hey there
I know the boat, I've not got one but a few folks have. There's two things you can do Carl? One you can buy a rudder from a model shop, they come in 3 sizes, you want the small one. Once you've got it thread it in the hole in the middle of the stern. Now there might be a raised area a little way from the hole, if there is I suggest it could be for the servo to e placed make 4 walls to house the servo stopping it from moving. Probably same model shop you get the rudder from will have the servo. A lot of folk are raving about micro servo's, yes they're good but not for what your servo is having to do. I realise it may seem very little but turning the boat while its moving is too much for a micro. On to of the rudder you buy is a bar with 2 holes , you want to put a bar on the servo don't panic it will be the bits which should of come with the servo. Once crewed on you want 2 stiff lengths of wire the length being from the servo to the rudder, Left hole in servo put one end of wire, to left side servo the same on right side. You've got a wire coming from the servo with a box shaped housing attached to the wire, this wire be pushed into the receiver I tend to use number 3, but 4 is OK but once boat is turned on move the right lever on the transmitter to find out which moves your best, i.e. the quickest response & then moves the furthest.
The other option is make a rudder out brass, your need a length of tubed brass & a flat piece How are you at soldering Carl? if you've not done it before then I'd suggesting buy your rudder & follow roughly what I've previously written

HI Damon, I quite like the boat as it but I want to fit servo and a rudder even 2 if that is what it supposed to have. I don't know if it is to scale but it is a good representation, as far as I can see, so don't want to put something on that is completely out of place. By the look of it at some point it had a manual rudder so there is a hole to make and mount one but the hole is right at the back. Which means it will be very difficult to attach Tilly (I think that's the name for it) and the rudder will stick out of the back of the boat. Not sure if that's normal so thinking of moving hole (or 2 holes 😀 ) forward to make it look better and get easier access? Unless of course the rudder/s are supposed to be mounted right at the back.

I'm still busy trying to get a smooth paintable surface on my Sea Queen so will not be starting just yet, just wanted some ideas, photo,drawing or plan, showing rear end if they exist.

HI Carl what type of boat are you making, how large, small is it finally how authentic do you want her to be? Just a rough guess on size will do, but an authentic vessel may also have double rudder? 😀

http://www.asrmcs-club.com/boatswebsite/hslhdinfo.html

Any hints on what rudder may look like?
Had a look on above website and only pics I can find, with boat out of water, I can see the three props but no rudder.