Your C64 may be old but the power supply, that’s another story.
Updated: Sep 19, 2022
So, the other day whilst (admittedly) working in a messy environment, I managed to fry some RAM chips in my C64. The reason? I accidentally plugged in the wrong power supply. My intention (as always) was to use my modern replacement and test the original one properly later, but it only took a tiny slip and I popped in the original PSU connector and (as expected) got a black screen on a previously working computer. This got me thinking about whether in reality, it's ever really worth using an “original” power supply and I came to the conclusion that it isn't. Now as an owner of a couple of modern replacements I can testify to a few things that might justify the investments I made. So, here goes.
First. Commodore 64 power supplies (especially) are not known for their reliability. Of course “back in the day” they did the job pretty well and as expected...but that was forty years ago and during that time the capacitors have often sat around for years and this dries them up. Now, the PSU might still function for a while but eventually one or more capacitors can explode and I can assure you (besides the shock) the smell is unbearable.