We do not carry smaller rear chain guards for the Razor MX500 dirt bike.
If the old chain has been used for a while then it is a good idea to replace it with a new chain when replacing the rear sprocket. We also recommend replacing the motor sprocket at the same time so all of the sprockets and the chain are new. This will help to prolong the life of the sprockets and chain.
We have a chain length calculator that can be used to determine what size chain is needed when a different size sprocket is installed.
If you need any help using the chain length calculator to determine the new chain size then please let us know.