First thing I would check is the clutches on the engine. From there, check all of the gears inside the trans and make sure the dog is swinging out on the input gear inside the trans. If it is not, either clean it really good and apply fresh silicone grease, or replace it. But it is normal for the spur to spin free, this is what allows reverse in these Rc cars. But when you rev the engine up, it should slam into gear, literally.
As for the forward reverse being enough, you will be able to find that out when you get it moving under its own power. But that may be one of the issues. There may be a stripped gear inside the trans. They are all plastic in there so it's likely if it wasn't taken care of.