Yep, a strip and clean is indicated. After 40+ years, lubricants can tend to get gummy, and crud builds up.

That would be one of the early Miroku made guns, by the sound of it. Wisners is the only supplier of springs for these that I know of.

You might want to read through their very comprehensive info page on Miroku's, as they do say that their replacement springs are not necessarily a drop-in, and some fitting may be required.

http://www.wisnersinc.com/additional_info/Mirokusprings.html