You make part a subassembly, go to your text editor, find SimplePlanes subassembly folder, the subassembly you need, and change specs. For example, trying to mod propeller engine, you save it as subassembly, find it as a file, change what you need, save changes, and go back to SP and paste your moded engine from subassemblys.
You are welcome @TehDuck. There are much better explanations on reddit. Google it.
Thanks for the info @MaleticAirplanes
You make part a subassembly, go to your text editor, find SimplePlanes subassembly folder, the subassembly you need, and change specs. For example, trying to mod propeller engine, you save it as subassembly, find it as a file, change what you need, save changes, and go back to SP and paste your moded engine from subassemblys.