I recently had a Polaris 120 in the shop. This is a mini snowmobile designed for kids. It uses a 120 cc "industrial" engine. This is a "Briggs and Stratton" type engine like you would see on a lawn mower or other outdoor power equipment. Arctic Cat and Skidoo also make 120 cc sleds and they are all put together the same way. There is a simple centrifugal clutch and a chain drive to the track. The chain drive is the biggest source of trouble on these machines, it needs to be lubricated and tensioned regularly.
When installing the chain a master link is often used. They come in a few different styles, but they all work the same. There is a chain link with a removable side plate and some type of clip to hold it together. These are quick and easy to install, but are also prone to failing. Over the years I have had several of them come off on different vehicles.
A more secure way to connect the chain is with a chain rivet tool. This tool is used to press the pins in and out of a regular link in the chain link. It takes a little longer to use than a master link, but will produce a more secure chain.