Old loco chassis

I went to put the TCS MC2 DCC decoder into the Watford Tank but realised the decoder I had was about 1mm too wide. Thankfully TCS have a different decoder, the M1, that I will purchase which will be ideal.

Flushed with the success of having a working chassis I got out another loco that I had been working on - the Judith Edge LMS/BR Jackshaft drive 0-6-0DE. I knew I could put the existing TCS MC2 decoder in the body of the diesel. However, the chassis didn't unscrew from the body cleanly and then I realised the chassis was all pitted.

At this stage I am unsure how much it will clean up or whether the damage has gone too far. I shall take advice and decide what possible cleaning can take place.