You need to perform a leak-down test on number 7. That will tell you if the compression loss is a valve or the piston. You seem to think it is a piston - how did you determine that? And I'll second the opinion that saving money is not an option. You're in for a major repair or rebuild, or a used engine.
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Question
What should I do I’ve got the code P300 on my 2015 Dodge Charger hellcat and I’ve bought new spark plugs and move the coil packs around… then i got the P307 code, I switched the injector from cylinder #7 to another cylinder same problem… cars sounds ruff while idling n so forth. I’ve got the compression test done and found zero compression on cylinder #7 said the piston has failed… what should I do???? Help me save money 
