What is the issue you are having? As far as compatible motherboards, when searching on the P500, it looks like the P500 uses a single socket 2011-v3 motherboard, so your existing CPU and RAM would likely work in another X99 board. That said, decent X99 boards are hard to find, especially at decent prices. Also, I am not sure if the motherboard layout is standard ATX, or something else, and I do not know if there are any proprietary connectors to the case or PSU. So if you get a new motherboard, you might need a new case.
What are your full system specs? If your P500 is having issues, or just isn't fast enough anymore, it may be wise for a full new system, or possibly just some minor repairs depending on the issue.