The VIC-II and SID chips in the C64/128 are prone to over-heating and are rather expensive to replace nowadays. Some other ICs in these machines may have the same issue.
An unmodified machine includes a large metal RF shield which usually performs the duty of being a heat sink for VIC-II, SID, and other chips. However, the RF shield is frequently missing from machines that have been damaged, worked on, modified, refurbished, etc. Therefore, that heat sink is also gone. I also question whether it was ever a very good heat sink to begin with, or just another way for Commodore to control costs.
What is a reliable modern replacement for the original RF shield's heat dissipation function? Which chips in the C64 and C128 should have such a heat sink installed?