The "trigger-cocking" mechanism on the Starr is relatively delicate, but most of the parts failures I've seen are due to folks trying to operate it incorrectly.
Generally, they've tried to operate it by thumb-cocking the hammer, something the design doesn't have any provision for. Doing so damages the action and ruins the spring that allows the sear and its releasing lever to engage the hammer.
The difference between the Starr's system and (for want of a better term) "conventional" DA mechanisms is that the hammer must be cocked via the trigger. All that little "selector" on the guard does is stop the sear release (the "real" trigger) from being tripped "automatically" at the end of the cocking stroke.
Briefly, to use the "SA" firing mode the "trigger" is drawn back until the cylinder has rotated to the next chamber and the hammer has been engaged by the sear. The trigger finger must then move to the sear lever (that little gizmo at the very rear inside the guard, the "real" trigger) and release it manually.
While you might be able to pick up a repro that somebody's broken in the $150 range, repairs aren't generally a DIY project unless you're highly skilled and intimately familiar with how the Starr system is supposed to work. Parts are expensive and hard to come by, and adding the cost of having a competent professional do the work would almost certainly erase any 'savings' over the retail cost of a new repro, and then some.
If you are prohibited from carrying a modern handgun, openly or concealed for whatever reason due to a statutory age restriction don't expect to be cut any slack whatsoever by any LEO or court for doing so with a C&B handgun. While they may not be subject to the same sales, etc. restrictions, they're still considered to be "deadly weapons" and subject to the same carry and usage rules as anyone's 1911 in most all jurisdictions.
I'd highly recommend that you choose not to open that particular can of worms.