For C-180's and other planes that have STC's to convert engines to different configurations, such as Pponk, Texas Skyways and Airplaines...are the STC's bough for the airframe you are putting the engine into?

An example would be, can I take a converted used engine from another 180 or 182 from these STC holders an put it into my airframe? Assuming everything of course is allowed per the STC for my airframe. And if I can do I need to repurchase an STC from the STC holder??? Or will the original STC work for a second airframe?

Not sure where to look for this information...

I'm no IA or paperwork wiz, but it seems to me that two stc's are needed for say a 470+50 Ponk conversion:
1) one to modify the engine, and
B) another to install that modified engine in your airframe.
Modification approvals are funny things. I have a field-approved 337 from 1993 for installation of a 470K in my 1953 180 in place of the original 470A. This engine is approved on the type certificate for this 180, however the mount leg brackets on the K were replaced with those off the A, to allow use of the original 1953 engine mount (that was documented on another, separate field-approved 337). The original dual exhaust system was retained, but modified for the 4-bolt flange connection on the K cylinders. An STC'd McCauley prop installation was also documented on that A-to-K 337.
For all I know, some mechanics and for that matter maybe some FAA inspectors would view the whole A-to-K engine swap as a minor alteration, requiring only a logbook entry, since the A, J, & K engine all all already approved for the 180 model on the TCDS. The people doing this mod on my airplane back in 1993 obviously did not.
FWIW, the copy of the McCauley STC (#SA388NW by North Sound Aviation) is marked "for use on Cessna 180 Nxxxxx s/n xxxxx only". As are some of the other STC's in my airplane's records -- for example, the one for BAS jumpseats,