While the tank is off, I’m replacing the throttle and idle cables. I lost the idle cable spring (or there wasn’t one) that goes in the cable guide.

Needed?
Can I use a spring from a ball point pen?

 

If you mean the return (push) cable, you don't need that cable anyway.

 

True, return cable really isn't totally necessary but that aside...
If a return cable is there, that stubby spring on the carb end needs to be there to allow proper adjustment and avoid things getting tangled on the end which is possible if it's missing.

If you buy new cables, the new return will have one installed. If it doesn't, they sent you an incorrect cable. It's also possible to "spiral" the old one off and back onto the new cable.