Ok, first let's decide what you're going to add, and what you're not.

If you put a smoker, 3 servos in the tail, hatches all over the place, and a bomb drop, that nice plane is goint to be too heavy to fly.

This thing is a fantastic airplane, and a 120 will be excellent in it, but DON'T go weighing it down with a ton of stuff.

A smoker is nice, I had one in mine, but it really pushed the plane to the weight limits. So I wouldn't add any more than 1 "Cool Mod"

Lightening is a great idea. I would thicken the wood on the tail feathers and skip the sheeting. A few lightening holes in the Fuse and ribs (Yes, the ribs) would help too.

IF you're going to add a hatch for the smoke pump, add it to the bottom right in front of the landing gear.

You can squeeze the smoke oil tank into the radio compartment right behind the fuel tank (Which doesn't need a hatch, you can get to it from the rear.)