I've been thinkin of getting back into CL flying after some 30 years of RC.
Towards this, I pulled out some of my old plans from 30-40 years ago, and was wondering: none of the models seem to feature any means of getting to the flap and elevator horns for adjustment. Is this still the practice today? is it beneficial to modify and add some access hatches or is it enough to ensure an accurate setup during construction (zero flap-zero elevator, and clevis location on horn as shown on plan).

The most sophistocated approach today is to make everything adjustable. There are many less sophistocated airplanes doing well, however. I make leadouts, tip weight, and tank location adjustable. Try to get the flaps and elevator as I want them. If you are just getting back, I'd suggest making the first airplane as simple as possible.

Thanks!
I'm very glad to have found this forum, thought CL was dead today, with all those cheap RC ARFs around...

It's alive and well to the extent there are now some CL ARF's around with things like the Nobler and some more up to date stunt designs. Almost all models now have some means of adjusting the controls ranging from just the elevator right up to everything possible. If you build a profile where all the controls are out in the open then the simplest way is to use the RC type control horns and adjust the flap and elevator to your heart's content . As Jim said though, adjustable leadouts are pretty much necessary and simple to make, same with a tip weight box unless you use stick-on mag wheel weights.

OK, so if adjustable leqdouts are almost a necessity, how do you adjust them. I have adjustable leadouts but have no idea how to go about optimzing the flight of my SCORE ARF.

Put them about where you think they should be, maybe 1/2 -3/4 inch behind the CG. Fly the airplane. If it gets light on the lines up high in the circle, move the leadouts forward 1/8 inch and try again. If it is Ok up high, move the leadouts back 1/8 inch and fly again. You will probably end up where you are looking right down the airplane's wing and you can see that the airplane is flying tangent to the circle, rather than yawed in or out.

It's a matter of trial and error.
The aim is to get the model to fly tangent exactly to the circle otherwise, if for example the lead out is too far aft, you'll get the model to fly with the nose out of the circle and pulling the lines very much, but loosing tension overhead, and vice versa, if the lead outs are too far forward, you loose tension in normal flight.

Also, theoretically at least, even if you have found an optimal position and are comfortable, if you change the CG position, and/or line thickness, you will need to change lead out position.

If I remember corectly (haven't flown these some 30 years or so), you start with a lead out position about 2 deg. aft of your CG and check during test flights.
BTW, the bellcrank position is not important here if I'm not mistaken, only CG and lead out positions affect the line tension.