The circuit traces are so wide, even if it's corroded, it can be repaired by wire bridging.
Take the tubes and transformers out for manual wiping with cloth and water/IPA.
The circuit board, Spray/hose the board with water and scrub with brushes and dish cleaner, no worry, you can dry the board later. Make sure all the surfaces rust, dirt and dust get scrubbed away. if you are fussy, the potentiometer and knob can be replaced. I will replace the turning knob, not that it's corroded, but to something majestic, big and elegant. Like a triangle knob resembling gryphon knob, but generally BIG :-)
Then dry the board with cloth, then spray with WD40, after wipe the board again and leave it for a day of drying.
As for the transformer and tubes, spray with WD40 on the surface and contacts and wipe off with cloth.
Some may not like WD40 because of smell, you can find one that contact clean the surface as well as prevent oxidation and protection. But I personally use WD40 since I just want to protect the surface for sure and avoid any future pain.
Then after repair any broken traces.
It didn't look that bad really, I have seen much worse than yours and still nicely repaired.