The only way you'll get authentic DK and DK Jr. is to buy the original boards. As long as you have the 2-board version of DK right now, the harness and connections for the DK Jr. board are exactly the same (there are 7 or 8 wires that connect to the board). To play DK Jr. you just unhook the connectors on the DK board and hook them up to the DJ Jr. board.
Also, Dokert and TimberTerror make an edge connector harness that replaces those 7 or 8 mini connections with one large connection, making it easier to switch between the boards. So, you can mount DK, DK jr. (and even DK 3) in the same cabinet and you can swap between the games by (1) powering down, (2) moving the edge connector from one board to the other, and then (3) powering back up. There is no widespread mod out there where you flip a switch or something to change between the boards.
There is a "double donkey kong" board modification. This is just a DK Jr. board modded to allow you to play DK on it. The problem is that DK boards have different analog sound circuitry than DK Jr. boards (DK Jr.'s analog sounds are pitched much higher). So when you play DK on a Double Donkey Kong board it sounds wrong.
None of the mulitcade boards accurately reproduce the sounds on Donkey Kong or Donkey Kong Jr. because they don't have the analog sound circuitry.