The D4990 is not an operator that is meant to be field serviceable. The parts that are available for replacement are as follows.
D4990 Arm: A45-501L Long Arm
*Please note the D4990 cannot be field converted to a D4990T by swapping arms and vice versa. The motor assemblies are different.
The Controller Board: 177609
Closer Body: DB4550-689
There is not a longer arm available for larger reveals than 6 3/4". This is the largest reveal the push side operator can accommodate. The track arm cannot be used on a reveal of any size.