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Needing advice on DOHC engine with low compression


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Hello looking for some advice on my 1981 cb 750c and it’s low compression. I adjusted valves after my initial reading of 62,65,72,75 psi on my compression test after doing valves numbers all jumped couple psi dropped little bit of oil into the spark plug holes and they jumped to 135,130,125,145 so my question is my google search on piston rings being my problem the actual problem and should I replace the pistons as well if I end up doing new rings looking to keep the same size pistons.
To make things more interesting I do have another 1981 cb 750 motor but she’s definitely seen her days should I mess with that see if I can get lucky or stick with the original motor. I acquired the motor as a deal with oem carbs I bought 100 bucks for carbs and a motor I have been told the motor used to ran before it was pulled and I didn’t get any more details. Any advice is welcome thanks in advance
I would take it apart first and look things over. Then you can see how much wear you have and determine
what all is required to do a proper rebuild.