Having the telescope focus is rather a critical part of being able to use it. Depending on the scope, they have many different ways of moving the focus tube back and forth. They tend to break fairly easily,  as the inexperienced operator tends to yank the eye-tube out of the barrel , often damaging the spur gear, rack or both. These 18th century rack mechanisms were always custom built and designed in different configurations. Ramsden used a slotted tube method, difficult to build and tough to replace. On this

example a majority of the slotted spur gear rack was completely stripped out, leaving a blank space for the gear to drop into and spin freely. The repair was made by cutting a section of new brass and soldering it into place. After reforming it to the barrel and polishing it to a slid fit with the draw tube, you then fixture it to  your milling machine and very precisely re-cut (+-.001″ ) the slot to match the movement of the spur gear as it   travels back and forth. Miss the math calculations and it will jam.

The telescope dealer doesn’t care. He asks two questions, “How much is it going to cost” and “When do I get it back”.  The collector is more interested in how the repair is done.