You can choose to go with a replacement, either the Kiwi upgrade or a clone pcb requiring all the related transfer of old components and addition of newer....lots of work!
However, don't give up yet... if you've already 'cleaned the board', you may have removed subtle signs of damage due to corrosion. However, you may yet find telltale signs of 'oxidation/corrosion', however subtle, on vias and upper pads.
In particular, as per a post by another member battling his intermittent/ issues for over a year or more - he was told to look in the area of IC18 pin 11 of the digital board (HD14051 analog switch).
I did the same, and other than slight oxidation, the solder joint at the pin (component side) seemed okay, but a continuity test to the related track via (~ 1 inch away) showed otherwise - a complete open.
I think this particular area is a point of electrical attraction to the (ionic/charged?) battery vapor... The pcb annular rings are very small and so it doesn't take much corrosion to affect the pin/pad connection, particularly as there's not much copper to begin with.
So...check the topside component pads in this same area for any tell-tale dullness etc., use a continuity meter to verify the traces. Touch up as needed, if the solder doesn't take, it's a sign you need to attach a jumper wire, directly on the pin, back to a good point/via (not too far). Don't forget to resolder the bottom pads too.
Good Luck and post any question if you're not sure....