The 4 to 1 is toast.
Is the rear input sliding gear the only gear broken? If it’s the only one broken I would try to get a free replacement gear, wouldn’t hurt to try. Elias sells tera low maybe they can help?
I know you’re frustrated with the gear set it is inferior to the lowmax. Lomax gear teeth are significantly larger and stronger than tera low and have machined single piece gears instead of two-piece gears but I would try to make the best of what you have whether you’re repairing it to reuse or resell.
As mentioned you could try and get a free replacement sliding gear/gears.
Another option would be to use the factory sliding gear. The original versions of the kit did not include sliding gears. They reused the stock gears and you could do the same. The sliding gear that comes in the kit is cut slightly different to reduce backlash but you could use the original in their place. I have an extra stock sliding gear if you need it.