Probably the easiest way is to buy an sp5k. Get a full auto pack from hkparts (yes they have all the stuff you need) register the metal trigger housing as a post sample MG cut the area that needs cut on the trigger housing to work on the sp5k host gun and there ya go. All the German with little to no fuss.
(Edit to add)
The sp5k has a nub in the back of the receiver to prevent a full auto carrier to be inserted. You'll need to either A: send carrier to be turned into auto carrier or B: buy an auto carrier for the sp5k. There are other ways as well