There are several ways to handle this. I will suggestion one.
Since you don't want to cut any of the new loom, make the following observations and changes. Leave all the old wires and cables on the original remote solenoid. Connect the old starter motor cable between the remote solenoid and the pre-engaged starter's threaded post. Connect a jumper wire between the pre-engaged starter's (battery in) threaded post and the larger of its two spade terminals.
What does this do? On later cars with the pre-engaged starters there is a starter relay your car will not have. When you turn the key on a later car it energizes the starter relay which passes current to the large starter solenoid spade terminal. That in turn actuates the pre-engaged solenoid and then spins the starter motor. In effect, by adding the jumper wire your old remote solenoid will simultaneously supply current to the pre-engaged starter's motor and its solenoid. Your old remote solenoid becomes both a current source and trigger for the pre-engaged solenoid.
See the picture in the link below (not my picture... just found it). Note the terminal identifications on the pre-engaged solenoid (left image).
http://s350.photobucket.com/user/shnevp ... s.jpg.html
This is the easiest way to use the donor engine and inertia starter. There are other ways.
