There are already several open x86 cores. You're not likely to find these as useful for modern code though, since most people are finally using x86_64 these days and a lot of the instructions being used (and a lot of the processor characteristics of modern chips, from SMT to the trace cache) are very different in modern Intel chips.

You might try instrumenting bochs, which might both run your code faster, require less hardware and give you more accurate results. In many ways, bochs is much better built for the type of thing you want.