Actually you can keep the crank.

If you go to your LBS, you can get some spacers for the cassette. With many cassette's you can break them apart and use the single one you need in combination with the spacers. Find out what gear ratio you prefer. Most people start at 2:1. IOW, if you've got a 34 T Chainring, use a 17 cog.

You can also remove your outer and granny chainrings. Move the spacers around to get the staightest chainline.

There is also a website out there that calculates all the combinations you can make with Chainring/Cog given the length of your chainstay, so you can eliminate the chain tensioner, but it usually easier and cheaper to just use your derailleur and lock down the set screws so it's fixed. If you set screws don't reach, you can use a small piece of cable and fix the derailleur in place.

The whole beauty of switching to a SS is using what you have, doing some dumpster diving or begging at the LBS for access to their junk box and keeping it cheap!

L8R