You can still buy most components for the C64, and failing that, there are a number of FPGA systems that reimplements it, as well as the C64 Reloaded reimplementation by Individual Computers [1] who have also licensed the Commodore name and is about to produce replacement cases.

Odds are that you'll still be able to replace a C64 with pretty much cycle exact replications long after you'd need to scramble for replacement for your current PC hardware...

[1] http://wiki.icomp.de/wiki/C64_reloaded