This is the closest free alternative. OpenTrack is an open-source project hosted on GitHub that allows users to input track geometry. While it lacks Kodub's polish, it offers a free plugin for Unreal Engine 5. Search for "OpenTrack Polysurface plugin" to get started.
Levi hit the gas. The car didn't just race; it flew. He wasn't fighting the game anymore. He was flowing with it. He drifted through the Poly-Corkscrew with a grace he’d never felt before. He drove backwards just to see the scenery. He pushed the car to speeds the developers never intended, the engine whining a high-pitched note of joy. kodub polytrack free
Physics-Based Stunts: Navigating loops, jumps, and wall-rides. This is the closest free alternative
He hit the first ramp at full throttle. The world blurred as his car soared through the air, performing a perfect barrel roll before slamming back onto the track. The suspension, slightly bouncy but responsive, absorbed the impact as he drifted around a sharp corner. Search for "OpenTrack Polysurface plugin" to get started