Here are some free resources you can use to build your own stages and cars
Rally Decor is a free plugin by Sander_rr that lets you easily add tapes to your stages
RallyNet is a free plugin for A-Chassis developed by Sander_rr which allows you to see your current time, penalties, distance and average speed. To install, put the RallyNet frame inside your vehicle's model > A-Chassis Tune > Plugins
Includes Wes Timing System, arrows, signs, spawner and a simple countdown to get started with making stages. Packaged by Sander_rr
Atrazine's Terrain Plugin - the most common way to create stages and terrain easily
Brushtool - Easily add trees, bushes, rocks etc. to your stages
Fence Creator - As the name says, you can make fences with it. Combine with SBS to easily customise them
Studio Build Suite - Super useful tool to resize, move and edit parts in ways the default Roblox tools don't let you
Archimedes - Create smooth curves easily