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After driving the 1st road test of about 1/2 hr, the engine would barely turn over. It was turning over “scaringly” slow. It did finally start.

Starter?
Battery and/or charging system?
Battery cables?

Did you try starting it again after it had cooled down, and if so was it better?

Sounds like the battery was not charging throughout the drive.

Do the usual charging system checks.

My first suspicion is the charging system. If the charging system isn't working, then the car is draining the battery to run. Restarting is going to be difficult at that point.

With the car running, put a multimeter on the battery posts and see what the voltage is. If your car is 6v, then it should be around 7v if the car is charging. If it's not, then you need to look into the charging system. Most likely the problem is with the generator. Thankfully they are pretty easy to open up and work on. Cleaning the commutator and installing new brushes is pretty simple.

Check ignition timing and all electrical connections. Has the starter been lubricated? 6 volt systems require a full size battery in good condition, and all starter and ignition wiring and cables need to be the correct gauge and in good condition. I have seen a number of cases where a worn or dirty starter will "drag" when hot. The ammeter should show a healthy charge rate above idle speed for several minutes after starting the car. Be sure the generator belt, if present, is clean and properly adjusted.

I'd stay far away from 12 volt batteries, 8 volt batteries, and dropping or ballast resistors.