When I built up my singlespeed, I used an inexpensive Dimension cyclo-cross crankset and narrow chainring bolts. It's made for two rings, but looks, and works, fine with just one.

I'm not sure what you want in a crankset. If you want something with more zoot, you can look at something like this.

Whatever crankset you choose, you will almost certainly need a new bottom bracket with shorter spindles. That gives you the opportunity to go with a splined interface, if you want. Or, you can go cheaper and stay with a square spindle bottom bracket.

Another thought: you could replace the outer chainring with a bashguard, eliminate the middle ring, and, if you have enough frame clearance, mount your single speed ring to the granny spider. That way you use the same crankset and bottom bracket, and leave no empty spider arms. I considered doing this, but just didn't like the look.

Whatever you do, use a chainring with no shift pins and ramps. Ditto the cog...use a BMX cog. Those shifting aids make your chain want to shift. On a singlespeed this is not good.