Hovis, I could very well be wrong, but I think the bike in your photo may be a four cylinder Belgian FN.(Fabrique National)

I have wondered about the durability of the skate board wheels for a long time.....are they functional on a bike that is really ridden?  Those tiny things have to spin awful fast when you are cruising down the interstate!

I was contemplating using Delrin, possibly having someone lathe down some round stock I have in the shape and specs of a skate wheel.

or something along the lines of the 80's model Husqvarna motorcycle tensioners.

All that modding, and he kept the stock tank...
I like the stock tank.  ...don't know why we knock it so much.
It's a nicely shaped tank.

Cheers to him for that.

The Husky tensioners were on long travel rear suspensions that developed a lot of slack and were on the bottom side. On the power side they won't last long. They got rid of the problem by moving the drive sprocket close to or on the swingarm pivot.
Bikes use tensioning pulleys or gears built-in to the derailleurs. They wear out after a few thousand miles and never have any real power or stress on them.
A pulley or slide on the rear drive chain is a patch or weak point fix to a problem that should never been designed into the drive train to start with.

Rat, On show bikes, it does not matter. Most of them will never be ridden 100 miles total.
On a bike you intend to actually ride, I would be very cautious about the engineering behind such mods. Your vid is a drag bike, most of which run rigid in the rear. The chain does not stretch. Either the chain is not properly adjusted, or their is flex in the system.
If the bike in the vid has suspension,  the chain is getting slack as it compresses. Perhaps jacked down to start with the swingarm straight with the front sprocket, and loosening as the suspension compresses. A good engineer would not design it that way. The tight point should be at the half way point. Or the swingarm pivot should be closer to the front sprocket.
Problems like this are what make radical conversions of m/c's so hard.(or fun)
Like trying to carry apples in a egg crate, you can do it,,,BUT, it's a lot easier to use a apple crate.

Using a spring loaded tensioner on a long travel conversion might be necessary sometimes, it would be messy(slinging chain oil all around) and a high maintenance item.
On a normal travel road bike their should never be a need for one.
If it was needed on a custom bike on the power side, the best bet would be a solid mounted sprocket. Enclosed preferably. And check the bearings often. But please don't even think about it on a drag, or high performance bike.

I came across the video of this build:

https://www.youtube.com/watch?v=qz4eNl3EV3A