Viable for what?
I have used my single speed for commuting 40km to work, century solo rides (ie 100km+ rides), fast 50km bunch rides, hell I have even raced it in time trials, criteriums and road races. OK 52x16 is not the best sprinting gear and any climbs over 200m and 6% are a PITA. But otherwise it just works.
Check the rear wheel, one side should be fixed the other side a freewheel. If not replace the fixed cog on the freewheel side with a freewheel ($25) an 18 tooth is a good place to start, and ride it.
If you need gears then look for a bike designed to use gears. Trying to retrofit a bike to use gears when it is not designed to, is going to challenging, costly and unlikely to be as efficient as a regular geared bike.