Kenwood sold the DK-520 kit to allow the TS-520 to work with the DG-5. Later models already had the VFO/HFO/BFO sockets on threar of the transceiver.
This link will show you how to build the simple buffers and where to tap into the TS-520's oscillators.
I did a similar thing for my Kenwood Twins and the AADE readout- worked beautifully.
http://www.n6wk.com/kenwood/DK-520-Install-Instructions.pdf73,
Dale W4OP