There isn't really a performance aspect of singlespeed.

There are external BB singlespeed cranks, SRAM S300 for example. You could also use your current cranks with a single chainring, you'd just need to buy singlespeed chainring bolts.

Only other thing that you'd need to do is either shorten your chain, or buy another chain (probably the better option if you plan on going back to a geared setup in the future).