Search eBay or AliExpress for "HP 2B34 Motherboard" or "HP EliteDesk 800 G1 TWR System Board" (Spare part number: 742472-001 ). Ensure you get the Tower version, not the Small Form Factor (SFF) version (which is a different board entirely).

While the i7-4790K is technically compatible, the H81 chipset does not support overclocking, so you will not be able to take advantage of its unlocked multiplier. Memory (RAM) Compatibility