Between all the interlocks and mode controls if you just want to spin up the Dynamotor think I would just use clip leads on the contactor coil , think its K-2703 or something like that. Not across the contacts but from +28 to one side of the coil and to ground on the other side. Think there was two or three relays that were involved in current limiting on one side and another contact between the relay coil and ground that was for PTT

Remember that when you do that you will have a bunch of HV that may want to go somewhere! Along with capacitors that will be charged too.

I have owned and did a couple AR-13 transmitters before but never used the dynamotor being that a AC power supply is way more behaved then the dynamotor, but it appears a lot of people feel that in order to get the maximum enjoyment one must experience the sensation of motor generated high voltage.

Ray F/KA3EKH