I converted my 1990's Rockhopper with vertical dropouts to a single speed years ago. Got by with a magic gear ratio for awhile till I didn't. Got tired of the chain popping off.

Could have just gotten a tensioner but there were issues with the original hub so I ponied up for an Eno Eccentric hub and had it laced to Velocity Aeroheat. Much better. Really is a nice, well built hub. Not cheap but they come up on eBay fairly often.