It lives!
No pics, yet, but I got the throttle and return springs sorted out. I also attached the seat back and cushion to the modified seat frame. I didn't have materials to fabricate brackets for attaching the seat frame to the sliding mounts. This was the progress I needed to attempt a test drive.
Attempt #1 failed because I didn't install the TQ's key stock.
Attempt #2 successful after installing the key stock. Under the cloak of night and illuminating my pathway via head lamp, I made a few flybys through the neighborhood. Of course, I made a few discoveries during the shakedown run.
1. a fuel drip revealed itself. Gotta figure out the source, obviously.
2. need to adjust the throttle cable or return spring. I have a typical go kart return spring on the pedal and a very heavy "found it in a bin" spring on the engine's throttle lever. With the pedal mashed, I can pull the throttle lever forward for more RPMs/speed. The throttle works fine, I could leave it as-is for the kids.
3. I have the pedals installed on the outside of the frame. The pedal will contact the tire under full throttle and hard left turn. I might install a steering stop to prevent contact, or figure out a plan B.