Bah, modding has always been a HUGE part of the C64 scene. It's one of the most popular PCs ever created and I'm pretty sure every museum or whatever who need a "pristine, untouched" original, already have one. Go nuts. 😀
My 64C has a dual-SID board fitted, stereo RCA audio jacks drilled (neatly!) into the outside of the case, a flash memory expansion, and a new-built PSU to replace the horrible system-killer epoxy brick thing they came with. Heaps of fun.
Also, it's really easy to peel up and put back that "warranty void" sticker. That's not a good indicator IMHO.
twitch.tv/oldskooljay - playing the obscure, forgotten & weird - most Tuesdays & Thursdays @ 6:30 PM PDT. Bonus streams elsewhen!