It is probably okay to thread modern wiring through the old lead tubing
in the Steck Duo-Art, but the opportunity for short circuits at the
junction points is kind of a red flag for me.  I recently removed all
lead sheathing from a Welte upright and replaced it with wire that has
rubber coating around plastic coated leads, and I included a ground to
the motor.

Also, rodents actually like to chew on lead to shorten their teeth.
I've had squirrels eat the battery posts off my tractors batteries.
Rodents are not particularly fond of rubber, but some plastic coated
wire, especially automotive wire, has a soy element in it, which can
attract the mice, rats etc.

So a three-wire cord coated with real rubber is what I decided to use
in the Welte.  Some really old wire had only the rubber coating with
the leads wrapped in cloth and paper, or another primitive "rubber"
full of sulfur and clay, any of which could cause the copper leads to
corrode.

Tim Gautreaux