Just made a huge noob mistake by buying a single speed road crank for my SS MTB buildup. I am converting an old rigid frame 90s MTN bike into a SS drop-bar, Townie cruiser and just bought a Sram 300 crank and Surly 18t cog. (Gain ratio 5.12)

Well, I just got schooled by the hipsters at my LBS and now I need a Mountain Bike Single speed Crank. Unfortunately they didn't know much about SS MTBs. My question is:

What MTB SS crank can I find that is most like a road crank? I see that MTB cranks tend to have longer arms and smaller chainrings than roadies so what is my best choice to get close to 48t, 170mm crank arm?
Otherwise, can I fit a 48t chain ring on a SSMTB crank spider?

Thanks,
Any help or suggestions would be very welcomed