This project has been slow and did not go according to plan, but I have had success so far!
The first thing to change was using the RK M1a as a donor for electronics. The tender board had a loose connection / wire somewhere, so performance was spotty and intermittent (see the sticky note on the board when I opened it up in the photos above). So instead, I cannibalized my MTH PS3 L1 2-8-2 for the electronics. It was a tough decision since my RK model was such a smooth-running model, but I'm changing over my roster from traditional/semi-scale sized engines to scale and I thought it fitting that the heart of my RK model would on in a scale sized version of the L1! I finished swapping the engine electronics today to the scale L1. While the installation is in no way elegant, I am happy to say when I plugged it all in and connected it to the MTH RK tender everything worked! The exception is the pilot mounted marker lights. I did not attempt to make them operable since they would only be used for reverse movements at night - not a typical scenario for my running. I will eventually install red / amber jewels to follow typical Pennsy pilot beam marker practice though. Pictured below is the model connected to the RK tender.
As for the tender, I considered just using the RK tender for a few reasons. First, the RK tender in not far off from scale. Second, MTH chose an oddball tender for this model that would not be typical for an L1 and third, it I could avoid the hassle of inserting the RK tender board into the new tender. However, the RK tender just does not look "right" to me so I will eventually swap the RK tender board into the new Premier tender. As for the new tender, installing the L1 shell on the USRA tender chassis took some work. The mounting holes were not aligned and the mounting posts in the L1 tender shell were inset almost 1/8" deeper in the shell. After some milling of the chassis and spacer additions, I am happy with the result. Pictured below is the comparison of the RK tender currently attached to the model with the L1 tender shell mounted on the USRA chassis to the right. 
This has been a rewarding project so far and was an economical way to acquire a Premier model, even when I include the cost of the MTH RK L1. I'm happy to add this new L1 to my growing scale-sized fleet!
