Same as any other system in that you have a choice
1) You recreate what a cheat engine would do and put a bit of code to constantly write to the location you found. Most people typically use the vblank routine
2) You find the thing changing your value (or all the things if multiple things reach out and touch it -- we do normally use the idea of lives for this one as an example if many things cause it) and change it to not do that (either a NOP, turning a subtraction into setting, stopping it from taking the less desired path).https://www.chibiakumas.com/6502/advanced.php
notes that the PCE/TG16 65c02 has inc and dec commands so less messing with the accumulator than other 6502 stuff might have.https://www.chibiakumas.com/6502/pcengine.php
is also a nice overview of the PCE hardware.
If you need a button combo for it to happen both of those can do it, though 1) might be easier there if you have limited space in 2).