Motherboard to fit Gateway SELECT 1000 C S

I am looking to upgrade my very old pc. I would like to use the current mid-tower case I have. This is a Gateway SELECT 1000 c s. I don’t know what motherboard is currently in it. It’s OEM b/c it’s a warranty replacement. The processor is an AMD Athlon, 1GHz and a whopping 128 MB RAM expandable to 256 MB. How can I tell (before purchasing a motherboard) that a new motherboard will fit?