These both came out very nice. Great job on the ROK MERDC paint scheme.
However there not too many aftermarket stuff, some PE and resin stowage, but no barrels...
As to AM for them, there is actually quite a bit, you just have to know where to look.
For the K9, DEF Model does a resin and PE update set that is quite extensive. It includes new rear baskets, a resin barrel and mantlet, new sprockets, new commander's cupola and weapons, etc. D-Toys also does a ROK Marine conversion which has resin, PE, and decals and includes a resin crewman as well. Orange Hobby has a metal barrel set and both Orange Hobby and MK1 Design offer PE sets for it. I don't know of any replacement track for the K9.
For the K1A1, Legend has an extensive resin stowage set that includes a whole new rear turret basket assembly full of gear. There is also a PE set from MK1 Design. The barrel for the K1A1 is the same barrel as an M1A1, an M256 barrel. Voyager, RB Model, and Orange Hobby all offer metal/resin M256 barrels. Trumpeter makes replacement, indi-link plastic T156 tracks for the K1A1.