The D-Boys KAC PDW uses a special type Medium motor since it has a shorter then usual grip. Normal medium motors (for SIG's) don't work.
I put a G27 grip on mine and used a long type motor in it. It looks great.
The D-Boys PDW will work with a normal type M4 hop up unit.
I'd reccomend re-wiring to the back and putting a battery pouch on for bigger battery space (did that to mine, works great), Re-Shim & Re-Grease, Metal Bushings (if it doesn't have them), G27 grip W/Matrix High Torque long motor (assuming your not going for a high speed set up), new HU unit, Madbull TBB, new 9.6V.