Figured this out! I hope this answer posted here helps someone else. The real topic here is "converting from a 3 to 4 wire motor.
First of all, a shout out to Solar Supply, Inc. in San Marcos, TX for matching the Mars 10204 motor with my GE for about 85 bucks.
The first thing he did when I got it was grab a capacitor and add it to the box. I thought, "I wonder why, I already have one?" Well, now that I compared both diagrams it makes sense.
The old motor's orange line was connected to one of 3 leads on the capacitor. From that line I noticed that there was a YELLOW line that went to the terminal block. I deduced that orange was analogous to yellow on the new motor. Black? That was easy, at least one thing was obvious.
That leaves the brown and brown/white pair. It finally dawned on me that these were to go on the
new, separate capacitor that the guy at Solar gave me. I'm assuming that having a dedicated capacitor for the fan motor is an overall improvement in safety and integrity. I suppose it would be theoretically possible to join the brown and brown/white wires to the old capacitor, but Murphy's law said that a 50% chance of getting it wrong is optimistic.. by so doing you're essentially joining one leg of the capacitor wires to the line voltage, and there must be a good reason why the capacitor is now separate.
The small green ground wire was a bit of a mystery but I just added it to the mount screw attaching the motor to the grill. Here is the diagram which should make everything clear: