Ok, here is my take based on the aircraft information. 1.5 pounds is a LOT of weight for an airplane that small. I would move much of the equipment as forward as possible to include servos even if I had to close off the "factory" holes. Moving equipment forward could include mounting the engine on "standoffs" to create the distance required. Changing to a different "heavier" engine is also an option. Bigger batteries, bigger servos, etc. I would try to exhaust all those options before adding dead weight to an aircraft.

I have built (from plans, kits and ARFs) many aircraft from the normal 40 size trainer, to my current 133-inch Citabria. I have yet to add more than a couple of ounces of dead ballast to an airframe. And as stated above, only add dead weight when I have exhausted all those options. I know that AMR is a reputable company, So I encourage you to look at the manual to make sure a step or component was not missed in the construction process. If not, that design could use some redesigning to make the nose forward of the wing a lot longer than it is now.

Rafael