I bought an extra shim and trimmed the inner diameter so I could fit it under the lip of the cylinder sleeve, and I installed it and removed a shim from on top of the sleeve to compensate, and I ran the engine today because the weather was relatively nice.

The top speed is exactly the same as it was originally. The engine is louder, but still not powerful enough to move the car faster. Considering that all these modifications accomplished nothing measurable, and considering that the engine revs much higher when the wheels are in the air, I suspect the engine just doesn't have enough power to push the car any faster, and changing the gearing would result in the car reaching the same top speed and taking longer to get there. The only other things I can think of at this point are to try increasing the compression and/or using a hotter plug to advance the ignition timing. I suppose trying a hotter plug would make sense, since it was about 50F outside today, whereas I initially tuned the engine in ~80F ambient temperatures. Or I could just wait until the weather gets hot again, and the ignition timing will advance on its own. I suppose it's noteworthy that the car achieved the same top speed in 30-degree cooler weather with a used glowplug and no significant re-tuning.

In my mind I know the engine must be better for all the work I've done, but maybe the car is just too heavy or un-aerodynamic for it.