I have put single speed kits on my multi speed mountain bikes before. Comes with cogs, tensioner and spacers (for cassette cog sets). I have not done one on free wheel cogs. Works well if you have the strength and stamina for it (I didnt).
Strip everything off except for brakes and go single speed with a free wheel, or remove the brakes too with a coaster hub. Get a nice set of cranks and I would uses thick slicks to help with rolling resistance.
I'm probably going to convert this 18" Prairie Breaker II to single speed. Add some aluminum XR50 bars and a layback seatpost.
I wouldn't mind a coaster hub. But that's an additional cost and I don't want to relace the rear wheel.
But I might not do it if I had to use a tensioner.
I got a similar 17" Raleigh M-40 that doesn't have adjustable dropouts and it will get a direct drive 32cc weedeater motor instead. It will reside at work.
Here's my old Schwinn Suburban that I stuck a Nexus 8 wheelset on. Deraileurs were kinda jacked up and I wanted to see how it looked with all that mess removed.
I thought it made the bike look way cleaner.
But it just didn't fit what I wanted. Too tall and bent over.
That hub, along with the Suburban's front sprocket and crank will go on my 26" cruiser/klunker build instead.