Simple solution

Just don't put it in and see how well it pulls.  Generally speaking, those old Athearns could pull quite a bit.  I would just try it and see how many cars it can pull without the weight.

Several variations

The blue box F7's had several variations.  The one you likely talking about is the "super powered" version.  (The others really had a lot of room in the shell.)  In this version, the weight fits over the motor and has a curved inside surface.  The curve was there to clear the old round motor and the flywheels.  The later rectangular motor was as tall, but not as wide.

These weights weigh around 170 grams of a total loco weight of 450 grams.  So leaving it out will be a bit of a loss in capacity.

The real estate between the weight and the underside structure is very tight.

For a DCC decoder, you could use one of the small flat decoders.  It is wired directly and does not require a plug.  It could sit on top of the rear truck tower.

Going with sound is another issue.  However, the speaker & decoder do have some mass.  You might need to cut the upper middle part of the weight away and replace it with the speaker.

Just some thoughts

Try it and see

I agree with skiloff, Try it and see how it pulls then if it needs more weight add some  lead somewhere. There's plenty of volume in an F-7 shell so you should be able to work something out. I try to not over weight locos, just the minimum needed to do their normal job, the spinning wheels act as a safety valve for the gears and motor if the train gets hung up for some reason.I'd rather run two half weighted locos than one heavy weight loco, much like the prototypes do on light trackage. .........DaveB