Hi aragorn723,
Repeated cleanings can help, of course, but successive treatments usually deliver diminishing returns. Citriclean, or other citrus-based cleaners can help - Caig makes one, but be sure to treat a small area to check for any adverse effects before treating the whole thing. The citrus aroma can itself be an issue; not everyone loves it.
I've had good luck with some low-tech approaches. Soap and water usually help to clean and deodorize grimy old electronics - be watchful of paper caps and other items that may be porous, dry thoroughly before connecting to power or operating. Sun and fresh air also help - expose as much of the amp as possible and literally let it 'air out' in the sun. Not sure why it works, but it can help. Note, of course, that UV exposure can be harmful to some plastics, possibly including some circuit boards, insulation, etc. Generally, the amount of exposure to air out an item won't be pushing UV exposure limits.
Ozone treatment frequently comes up for odor removal, particularly for smoke damage. I see that one can rent or purchase their own ozone generator, or hire treatment companies that do on-site or in-shop treatment.
