Looks have no real connection to processor choice.
Motherboard selection starts with the components that need to plug into it, or you start with a motherboard and get the appropriate parts.
The broadest selection for a motherboard is likely size. Mini-ITX, MicroATX, ATX, FATX, E-ATX, etc. Not much point looking farther if it won't fit in the desired computer. The two more specific things to define a motherboard that you need are the CPU/CPU socket and the type of memory supported.
If you are looking at working on an OEM computer, it can be desirable to get a refurbished or identical motherboard to the one that needs replacing. Many OEM chassis have special brackets or wiring, that while still conforming to ATX standards, can be difficult to work with.
One simple solution is to replace the case along with the motherboard, then there shouldn't be any compatibility issues.