Dolby HX PRO implementation on existing tape recording deck
I don't know if your question received a proper answer and the problem is solved or it is still actual. 

Anyway, your Fostex already appears to have an interesting bias loop control, due those parallel FET working as controlled resistors by a tension coming from an upper stage that is not full displayed. It is possible that this circuit to do a kind of HX Pro job or it is possible to be convinced to do, as long as the matter of tension controlling of bias is already in place. To adapt the uPC 1297 to your tape recorder, the main issue is the transformer from uPC (pins 11 and 12) to Fostex recording head. If the recording heads are to much different as Z, the transformer should be modified. 

Also, the input in uPC from the bias oscillator should be reduced by a divider to the proper amplitude. Something like this should be also done with the bias modulation signal coming trough 150&nbsp;k&Omega; resistor. This resistor with the 560&nbsp;pF condenser make a low pass filter you should keep the time constant in order to trigger in the same way the uPC. 

Give me the type of your Fostex and the whole diagram. I can look about service manual for some specs of recording head. Sometimes I use to enjoy some circuits math and to check with OrCAD the effects.