Could be the starter is getting tired is why it cranks slow when hot, and if you know the battery is getting old it may also be supplying less amps when hot.
I would keep the 6 volt. Have the starter checked out, cleaned, rebuilt if needed and make sure the 6 volt battery is doing its part.
The volt systems do work well when in good shape.
Also check the timing on the engine and points just in case there is some wear. Sometimes that will make one hard to start when hot.
