Unfortunately this item is expected to be out of stock from Razor Scooters USA for an extended period of time. If the broken part of your Razor E300 chain tensioner assembly is a broken spring, please see the item page for the chain tensioner spring at the link below:
OEM replacement chain tensioner for the Razor E300, versions 36 and up only.
Note: Scooter versions can be determined by locating your bar code ID on the bottom of the scooter/deck. The 7th and 8th digit in the series will tell you the version of your product: 01=version 1; 02=version 2; 09=version 9, and so on.