The one in the video was discontinued but here is a link for 2 other similar ones.
1" diameter engine shaft: https://amzn.to/3fIofkf
3/4" diameter engine shaft: https://amzn.to/3uLqbgr
Here is a quick video showing the torque converter setup install. You can find these TAV torque converters all over the internet. They are cheap and easy to install. If you are going to be using your mini bike or go kart a lot or you don't want to take it apart very often then I would suggest getting a Comet brand torque converter. Yest they are more expensive but worth it in the long run. Do you want a Harbor Freight tool or Milwaukee! If you look carefully you'll get a sneak preview of another yerf dog go kart i'm restoring and rebuilding.